Build suddenly has 'No Agents'

I have "Build A" with 8 dependencies.  If none of the dependencies need to be rebuilt, Build A usually starts right up.  However, if one of the dependencies needs to be built, I've seen twice where the Build Queue reports that Build A has no compatible agents.

Ironically, the Compatible Agents tab says:

• Compatible agents (2)
• Incompatible agents (0)

However, the Build Queue still reports:  Can run on:  No Agents

Does anyone know why this would happen?  Usually, if I just Run the build again ... it will work.

Thanks!
Bryan

Edit: I'm using TeamCity Enterprise 7.1 (build 23907)

Edit: This thread looks similar to what I'm seeing:  http://devnet.jetbrains.net/message/5461267#5461267