I recently found that if files are checked out in the workspace defined by the plugin. That even with clean copy set, TFS will tell you it cannot pull down new source. This though, somehow returns as successful meaning the restof the build continues on
normally. This condition should fail the build. The only way to figure out what happened is to go through the build log and find the message that tells you what happened. We were building the wrong solution for weeks, until we found that features we expected
in the build were not present and started digging.