I suddenly started receiving the message "Source not available" for a .prg when stepping through executing code.

To make sure I wasn't having a version conflict or anything like that, I created several .prgs, each with a unique name, placed each in a separate subdirectory that participates in the project and modified some code to call each prg in turn. Each executed, but the the source was not available.

I'm totally stymied--I've never had this problem before. Any ideas?
The only thing that changed on my system was that last night I installed Diskeeper and ran it to defrag my hard drive, but I doubt that could have affected anything. I am seeing the source for classlib methods.

thanks in advance, Chaim