Visual Studio won't load source or debugging symbols for mex file

I'm trying to debug a C mex file using Visual Studio 2010. I can load my source file and stop at a breakpoint in the main mexFunction entry routine, but if I try to step into a subroutine or continue to a breakpoint in a subroutine, VS says it can't load the symbols or source code. The subroutine is contained in the same .cpp file as the mexFunction routine. I can step into the subfunction using the yprime.c example, but can't do it for my mex code. Any ideas?