yield return on WWW never coming back?

I'm making a web hit with the WWW class. Something I have done a dozen times in other places but for some reason, although the hit to the server hits and comes back, the Unity client never picks the response up. I'm wondering if it's something quirky about WWW that I just haven't experienced before. I did some searching but everyone else having this problem is just because they didn't use StartCoroutine when initiating the communication. This isn't my problem.

Code calling method that yield returns the WWW object.

 StartCoroutine(_director.ProcessInstant(new Battleschool.Common.Entities.Contract.InstantDetail { AbilityId = _ability.AbilityId, GameId = BattleStatics.Instance.GameId, RoundIndex = (short)BattleStatics.Instance.RoundIndex, SourceId = _sourceCharacterData.CharacterId, TargetId = _targetCharacters.First().CharacterId }));

The method making the hit itself.

 internal IEnumerator ProcessInstant(InstantDetail details)
 {
     WWW www = new WWW(DirectorHelper.GetUri("Battle/ProcessInstant"), DirectorHelper.PrepareBody<InstantDetail>(details), DirectorHelper.GetHeaders());
 
     yield return www;
 
     // execution never makes it to this point
     if (OnProcessInstantComplete != null)
     {
         OnProcessInstantComplete(this, DirectorHelper.DeserializeAsBase64FromNetwork<IInstantResponse>(www.text));
     }
 }

Here's the response coming back from the server as captured by Fiddler.

 HTTP/1.1 200 OK
 Server: ASP.NET Development Server/10.0.0.0
 Date: Sat, 07 Feb 2015 16:34:20 GMT
 X-AspNet-Version: 4.0.30319
 Cache-Control: no-cache
 Pragma: no-cache
 Expires: -1
 Content-Type: application/json; charset=utf-8
 Content-Length: 686
 Connection: Close
 
 "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"

Appreciate any insights into this.

-=-=-UPDATE-=-=-

Incidentally, I moved to a completely synchronous approach by removing the StartCoroutine and changing the method making the hit to the below and it works fine. Of course, this can't serve as a solution, but thought it was interesting.

 internal void ProcessInstant(InstantDetail details)
 {
     WWW www = new WWW(DirectorHelper.GetUri("Battle/ProcessInstant"), DirectorHelper.PrepareBody<InstantDetail>(details), DirectorHelper.GetHeaders());
 
     while (!www.isDone) { }
 
     //yield return www;
 
     if (OnProcessInstantComplete != null)
     {
         OnProcessInstantComplete(this, DirectorHelper.DeserializeAsBase64FromNetwork<IInstantResponse>(www.text));
     }
 }