On TFS you need to check-in the whole solution and not just the project because if you check-in just the project then the dlls will not be checked-in.
You can also get Visual Studio Team Foundation Server 2012 Power Tools http://visualstudiogallery.msdn.microsoft.com/b1ef7eb2-e084-4cb8-9bc7-06c3bad9148f this will allow you to see outside of Visual Studio in your folders if you have checked in the dll or not.
Good luck!