A precondition has blocked the start of the build?

TeamCity Enterprise 7.1.3 (build 24266)

My builds were sitting in the queue and not executing (with several available agents.)  The debug log reported the following:

[2013-01-13 00:25:42,592]  DEBUG - ains.buildServer.util.WatchDog - Flush Queue start: 0 msec
[2013-01-13 00:25:42,592]  DEBUG - serverSide.impl.BuildQueueImpl - Queued builds selected for merge operation (personal and starting builds excluded): bt154 (promo id: 102723), bt161 (promo id: 102725), bt175 (promo id: 102724), bt180 (promo id: 102726), bt191 (promo id: 102727),  
[2013-01-13 00:25:42,592]  DEBUG - serverSide.impl.BuildQueueImpl - None of the queued builds can be merged
[2013-01-13 00:25:42,592]  DEBUG -   jetbrains.buildServer.SERVER - Compatible/can run on agents for: QueuedBuildImpl{myBuildType=<removed> {id=bt154},ItemId=102723} are: [kushtaka 27 registered: true authorized: true, iara 20 registered: true authorized: true, gulon 19 registered: true authorized: true], already distributed agents: []
[2013-01-13 00:25:42,592]  DEBUG -   jetbrains.buildServer.SERVER - A precondition has blocked the start of the build: QueuedBuildImpl{myBuildType=<removed> {id=bt161},ItemId=102725}
[2013-01-13 00:25:42,592]  DEBUG -   jetbrains.buildServer.SERVER - A precondition has blocked the start of the build: QueuedBuildImpl{myBuildType=<removed> {id=bt175},ItemId=102724}
[2013-01-13 00:25:42,592]  DEBUG -   jetbrains.buildServer.SERVER - A precondition has blocked the start of the build: QueuedBuildImpl{myBuildType=<removed> {id=bt180},ItemId=102726}
[2013-01-13 00:25:42,592]  DEBUG -   jetbrains.buildServer.SERVER - A precondition has blocked the start of the build: QueuedBuildImpl{myBuildType=<removed> {id=bt191},ItemId=102727}
[2013-01-13 00:25:42,592]  DEBUG - ains.buildServer.util.WatchDog - Flush Queue done: 6 msec

What does "A precondition has blocked the start of the build" mean?  How can I fix this?

Thanks,
Bryan