Why do symlinks sometimes vanish in Xcode?

The source files of interest are on a network share hosted on a Win7 machine, but the symlinks are created on this Mac using ln -sf

These are .h files, and sometimes one or more of them are not found by the C++ compiler. I get an error like foo.h is not found, when I can see it and read the (linked-to) contents using Finder.

My work-around is to copy the file instead of symlink to it. Then I find that the other files (symlink .h files in the same directory linking to files in the same source directory; just like the "bad" one) have no problems.

Which one(s) are affected will vary, too.

Any idea what could be causing this? It started when updating Mac OSX to Mavericks, which has different CIFS network code. It still happens with Yosemite.