If I see this problem it is usually caused by the way my source code is mapped in the project.  Are you debugging the 
source code from the same path you built it on?  If not you will need to set up a mapping in the debugger session 
options.

If you are debugging from the same path the code was built on, then make sure you have selected "Use full file path to 
set breakpoints" in the Debugger tab of Debugger Configurations.  Also, you should check out the Source Lookup Path 
setting on the Source tab of Debugger Configurations.  For my project I have to use "Absolute File Path" there, but it 
may be different for you.