TFS Agent Side Checkout Issue

Hi

We have 6 agents running around 60-70 build configurations using TFS 2010.  They are all setup to share the same VCS Root and use the "Automatically on agent" VCS checkout mode to a Custom Path on the build agents.  Beneath the VCS Root we have separate directories for the Mainline and different branches in TFS.  Different builds are setup against different branches and also the Mainline.  

We are running the latest 7.1.2 build of TeamCity and I've noticed that our builds  are taking a lot longer to complete.  On investigation it seems that the TeamCity is frequently performing a clean checkout on the different agents:

[16:18:25]Checking for changes

[16:18:26]Publishing internal artifacts (3s)

[16:18:30][Publishing internal artifacts] Sending build.start.properties.gz file

[16:18:26]Clearing temporary directory: C:\BuildAgent\temp\buildTmp

[16:18:26]Checkout directory: C:\TC\OKSource\OnKey

[16:18:26]Updating sources: agent side checkout (11m:43s)

[16:18:26][Updating sources] Will perform clean checkout. Reason: Agent doesn't have any version of the project sources

[16:18:26][Updating sources] Cleaning C:\TC\OKSource\OnKey

[16:23:01][Updating sources] VCS Root: OKSource_OnKey (7m:09s)

[16:23:01][VCS Root: OKSource_OnKey] revision: 78239

[16:30:10]Starting: C:\BuildAgent\plugins\dotnetPlugin\bin\JetBrains.BuildServer.MsBuildBootstrap.exe /workdir:C:\TC\OKSource\OnKey /msbuildPath:C:\Windows\Microsoft.NET\Framework\v4.0.30319\MSBuild.exe

[16:30:10]in directory: C:\TC\OKSource\OnKey

[16:30:18]Main\Pragma.OnKey5.proj.teamcity: Build targets: BuildDatabases;BuildAndTestOnKeySdk (6m:27s)

[16:30:18][Main\Pragma.OnKey5.proj.teamcity] GetPaths

Why is this happening?  To prevent this from constantly occuring, I tried the "Clean source on this agent" option for the 6 agents.  However, when I selec the option it tells me "There are no build configurations on this agent to enforce clean checkout for".  This is even more confusing as the agent has successfully build multiple of the 60-70 configurations in its recent history.  How do I go about troubleshooting the issue to find out why TeamCity is constantly doing a clean checkout?

Thanks