Some assets not loading in an old Rails 3-2-stable app - ruby-on-rails

Question

BIG UPDATES AT THE BOTTOM OF THIS POST
I have a Rails 3-2-stable app that I'm moving to a new server so I can work on upgrading it to Rails 5. Currently, it's hosted on Heroku and most of the assets show up just fine (the app Just Works (TM)). I have a few random assets that will not show up after precompilation on a Digital Ocean droplet. I'm deploying with Capistrano.
The error for the assets that won't show up look something like this:
Loading failed for the <script> with source “http://stage.fancy-new-droplet.com/assets/application/application.js”.
In some cases, there will be a hash associated with the asset like this:
Loading failed for the <script> with source “http://stage.fancy-new-droplet.com/assets/application/certifications-56046476595984b00d1267a4f02822e5.js
In the former case, the asset is actually in public/assets/ as opposed to public/assets/application/ on the droplet. Moving or symlinking the missing asset to that location doesn't seem to affect anything. In the latter case, the assets just don't exist anywhere on the droplet as far as I can tell.
For the case with no hashes in filenames, changing my include tags from this:
<%= javascript_include_tag "application/application" %>
To something like this:
<%= javascript_include_tag "application" %>
Will solve the issue but I feel like that's a red herring given the other assets that don't seem to exist after precompilation.
I did run into an esoteric issue where the droplet was running out of memory and silently failing halfway through asset compilation but I increased the available memory and it doesn't even use half the memory now. It does max out the CPU 100%, though. I may try to increase both CPU and memory on the droplet.
Here are the relevant asset compilation lines from config/environments/production.rb:
config.serve_static_assets = false
config.assets.compress = true
config.assets.compile = true
config.assets.digest = true
And the relevant asset compilation lines from application.rb:
config.assets.enabled = true
config.assets.version = '1.0'
Locally, everything works 100% just peachy, no errors. Assets load fine and we're all good. Here's the asset pipeline bits from my development.rb file:
config.assets.compile = true
config.assets.compress = false
config.assets.debug = true
I've run various combinations of rake assets:clean, rake assets:precompile, and rake assets:precompile:all on the production server (in the production environment) and I've even nuked the public/assets/ folder to start it all over again. All to no avail.
What might cause these assets to not be created on our droplet while they do manage to get created on Heroku?
EDIT
As requested, my DO stack is simply just a droplet (2vcpu, 2gb ram), I'm using Puma and Nginx. Here is my Capfile:
# Load DSL and Setup Up Stages
require 'capistrano/setup'
require 'capistrano/deploy'
require 'capistrano/scm/git'
install_plugin Capistrano::SCM::Git
require 'capistrano/rails'
require 'capistrano/bundler'
require 'capistrano/rvm'
require 'capistrano/puma'
install_plugin Capistrano::Puma
# Loads custom tasks from `lib/capistrano/tasks' if you have any defined.
Dir.glob('lib/capistrano/tasks/*.rake').each { |r| import r }
FURTHER EDIT
I've frobbed config.assets.initialize_on_precompile a couple of times in application.rb and ran various permutations of rake assets:clean, assets:precompile, and assets:precompile:all with RAILS_ENV=production locally to no avail.
YET ANOTHER EDIT
I've noticed at this point that precompiling production assets seems to not be compiling four of the javascript files, and about seven of the css files. It also doesn't seem to put application.js into public/assets/application/application.js (where the app is looking), rather it lands in public/assets/application.js.
MORE EDITS
I have resolved the issue of the JavaScript assets not loading by fixing a require_tree statement in application.js and now the only issue seems to be several missing CSS files.
In this application there are two directories with css files:
app/assets/stylesheets/
app/assets/stylesheets/application
There are a total of 21 stylesheets that do not load out of nearly 100 total. The stylesheets that don't load are being called with a hashed filename (somefile-hash.css) but those hashed files don't exist in public/assets/ on the droplet.
Here's the requires from application.css.scss:
/*
*= require_self
*= require chosen
*= require jquery-ui/autocomplete
*= require jquery-ui/datepicker
*= require feature-carousel
*= require_tree .
*/
Out of frustration, I added = require_directory ./assets to that list and now I've only got about 15 stylesheets failing to compile/load. That behaviour doesn't make any sense to me as I assumed the = require_tree . would've covered the wonky subdirectory.
Individually specifying files = require foo or = require foo.css.scss does not seem to affect anything, either.
I also bumped the config.assets.version out of frustration to no avail.
And furthermore, no amount of foul language can persuade these assets to compile.

Answer 1

The asset pipeline concatenates your assets to only generate and serve specified files, 'application.js', 'application.css' and images/fonts by default.
If you are explicitly including other css/js files in your application via the javascript_include_tag or stylesheet_link_tag helpers, you'll need to add those files manually to the pipeline via the config.assets.precompile += setting in your application.rb or an initializer.
Currently, those files should already be included within your application manifest via the require_tree . directive. So, the styles might be being applied right now too. If that's the case, you can just remove your code that adds the individual stylesheets separately to get rid of the 404s.
Still not sure how the app is working fine on heroku, maybe you have some heroku config that's enabling serving the static assets.

Answer 2

Perhaps try changing these in production.rb but no guarantees, may be worth trying.
config.serve_static_files = true
config.assets.compile = false
Also if you have this in Gemfile, you might wanna try to comment it out and bundle install locally before deploy.
gem 'rails_12factor', group: :production
That 💩was required by Heroku but you may not need it on DO. See https://github.com/heroku/rails_12factor
Also, please if you can post more relevant info about your DO stack and maybe your Capfile

Answer 3

Just changing this on your production.rb should work:
config.serve_static_assets = true