Windows 7 to XP file sharing: some files are accessible in a folder,some are not

I have a Windows 7 desktop that's sharing several folders out to other
computers, mainly running XP. That's pretty typical, and I'm not having
any problem setting this up, it's already setup and working fine.

However, recently I added a new folder share from the Windows 7
computer. What I'm finding is that some of the files in the folder are
accessible, and some are not. It's unusual because typically I see
either that the whole folder is accessible, or it is not, not just some
things inside it. I'd say half are accessible, and half are not.

When trying to access some files, I get the message: "file not
accessible". Yet, I can see all of these files when viewing the
directory listing in Explorer. What could be causing this?

Yousuf Khan