Using cocoon gem, but can't save data after first input field - ruby-on-rails

Question

I've tried to implement cocoon in my application, but having trouble saving data after the first input field.
I tried following the example, but no luck.
I'm able to save the first field into database, but nothing happens on the other fields when I add more.
_form.html.erb
<%= simple_form_for(#project) do |f| %>
<%= f.input :project_name %>
<%= f.hidden_field :user_id %>
<div id="tasks">
<%= f.simple_fields_for :tasks do |g| %>
<%= render 'task_fields', :f => g %>
<% end%>
<%= link_to_add_association 'add task', f, :tasks %>
</div>
<%= f.button :submit %>
<% end %>
_task_fields.html.erb
<li class="control-group nested-fields">
<div class="controls">
<%= f.label :task %>
<%= f.text_field :task %>
</div>
<%= link_to_remove_association "remove task", f %>
</li>
Controller
params.require(:project).permit(
:user_id, :project_name,
tasks_attributes: [:id, :task, :_destroy])
And I added:
//= require cocoon
in application.js
Project Model
class Project < ActiveRecord::Base
belongs_to :user
has_many :tasks
accepts_nested_attributes_for :tasks, :reject_if => :all_blank, allow_destroy: true
end
Task Model
class Task < ActiveRecord::Base
belongs_to :project
end
I think I got this correct? I'm able to click on "add task" link and a new field pops up, but those new fields doesn't save.
EDIT from POST in console
Processing by ProjectsController#create as HTML
Parameters: {
"utf8"=>"✓",
"authenticity_token"=>"blah",
"project"=>{"project_name"=>"cocoon test",
"tasks_attributes"=>{
"0"=>{"task"=>"fix this!",
"_destroy"=>"false"
}}},
"commit"=>"Create Project"}
(0.2ms) begin transaction
SQL (1.5ms) INSERT INTO "projects" ("project_name", "created_at", "updated_at") VALUES (?, ?, ?) [["project_name", "cocoon test"], ["created_at", "2015-07-15 03:26:09.444377"], ["updated_at", "2015-07-15 03:26:09.444377"]]
SQL (0.4ms) INSERT INTO "tasks" ("task", "project_id", "created_at", "updated_at") VALUES (?, ?, ?, ?) [["task", "fix this!"], ["project_id", 8], ["created_at", "2015-07-15 03:26:09.450324"], ["updated_at", "2015-07-15 03:26:09.450324"]]
(8.4ms) commit transaction
Redirected to http://localhost:3000/projects/8
Completed 302 Found in 24ms (ActiveRecord: 10.4ms)
EDIT 2 posting project controller methods create and new
def create
#project = Project.new(project_params)
respond_to do |format|
if #project.save
format.html { redirect_to #project, notice: 'Project was successfully created.' }
format.json { render :show, status: :created, location: #project }
else
format.html { render :new }
format.json { render json: #project.errors, status: :unprocessable_entity }
end
end
end
def new
#project = Project.new
#project.tasks.build
end