I've noticed this problem in the past but it seems to be coming up all the time now. Here's what I do:
1) Run All Tests in Solution from the menu (I'm using VS2010, NUnit, R# 5.0) -- close the Unit Test Sessions window by clicking the X in the top right corner (not the X on the tab)
2) Add a new test to an existing file
3) Run All Tests in Solution from the menu again
At this point it runs the tests that were already in the Unit Test Sessions window, but not the new tests that I added. The next time I do Run All Tests in Solution, it will pick up the new tests.