Visual Studio 2010 does not discover new unit tests

I've had this problem as well when building in debug mode. When I switched it to build in release mode that fixed it for me.

I had a similar problem in Visual Studio 2012, however none of the solutions worked. I am developing a 64 bit application and the solution was to choose:-

Test -> Test Settings -> Default Processor Architecture -> X64

I finally figured out the problem. In Options under Test Tools -> Test Project, there is an option "Disable background discovery of test methods". It was turned on - I do not know how that happened, but removing the checkmark and restarting Visual Studio solved the problem.