request.GetClientCertificate Returns null on Specific Server - asp.net-mvc

Question

I'm using client certificate authentication in an ASP.NET Web API 2 application. I attach the cert to the client like so:
private HttpClient GetHttpClient()
{
HttpClient client = new HttpClient(GetRequestHandler());
client.BaseAddress = new Uri(_apiBaseUrl);
return client;
}
private WebRequestHandler GetRequestHandler()
{
WebRequestHandler wrh = new WebRequestHandler();
X509Certificate cert = GetClientCert();
wrh.ClientCertificates.Add(cert);
return wrh;
}
private X509Certificate GetClientCert()
{
X509Store certStore = new X509Store(StoreName.My, StoreLocation.LocalMachine);
certStore.Open(OpenFlags.OpenExistingOnly | OpenFlags.ReadOnly);
X509Certificate cert = certStore.Certificates.Find(X509FindType.FindBySubjectName, _certName, false)[0];
certStore.Close();
return cert;
}
When I debug this, I can see the cert object is what I expected. In the Web API on the server, I'm using this:
public class CertAuthHandler : DelegatingHandler
{
private ICertValidator _certValidator;
public CertAuthHandler(ICertValidator certValidator)
{
this._certValidator = certValidator;
}
public CertAuthHandler() : this(new LoggingCertValidator()) { }
protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
var cert = request.GetClientCertificate();
if (_certValidator.IsValid(cert))
{
return base.SendAsync(request, cancellationToken);
}
return Task<HttpResponseMessage>.Factory.StartNew(() => request.CreateResponse(HttpStatusCode.Unauthorized));
}
}
This works exactly as expected on our TEST server; however on our DEV server the logging that I added for troubleshooting this shows that request.GetClientCertificate() is returning null. I double and triple checked that IIS is set up to accept client certificates; when I change it to require ssl and require client certs it returns a 403. If it matters, the Web API is set up as a sub-application of the main website in both DEV and TEST.
I also had some teammates try the client from their machines to make sure it wasn't something to do with how the cert is installed on my machine (we are using different certs for DEV and TEST); they also get a 401. I've used the same DEV cert to call another API with the same security setup, and it works without issue, so it seems to be server-specific. My question is, what could I look into on our DEV server that might be causing the cert to not come through with the request? I'm not able to find anything related to this in the server's event logs, with the exception of my own custom logging that I added to the API. Any help is greatly appreciated.