Why can I not enable breakpoints in Visual Studio 2005?

I have seen that sometimes breakpoints cannot be enabled in Visual Studio, they appear as empty circles, with a warning sign when enabled. This icon change is all the information VS gives.

Particularly I'm suffering开发者_运维百科 that in a Windows CE 6.0 project where I cannot enable any breakpoint in any of the sub-projects or in the OS design. The target platform in this project is a Vortex x86 embedded board, I use KITL for remote debugging.

How can I retrieve more information about what is happening? Is it not possible to use breakpoints in my case?

The hollow circles with the yellow warning sign are OK. The breakpoint should instantiate once the module that holds that breakpoint loads.
This of course happens in a debug image only.

Are you running/compiling a debuggable version of your projects - I seem to remeber if there were no symbols loaded then this was the casE?