2nd network computer can't see 100% of files in certain folders on 1st computer

I have a 2-station wireless network between my office and my secretary's,
next door. I have an XP Pro machine; she has XP Home. Both computers are
reasonably up-to-date and otherwise working well. Both have fairly recent
D-link hardware. I have had the wireless network for over a year, most of
the time working well. Recently, a very odd problem has popped up.

My secretary's computer sometimes cannot see all the files in a particular
folder. The problem has shown up in several folders during the past few
weeks. Since her job is to finalize and prints documents that I draft,
this has created uncertainty, since she does not know about all the
documents she is supposed to process.

I am pretty sure I have sharing working properly. She can access my
computer and can see, for example, 11 of the 13 files in, for example, the
"Jones" folder, but not the 12th and 13th. The missing files seem to have
nothing in common., that is, they are random, as far as the date is
concerned, etc. They are not unusually large or small. All are text
documents, drafted in WordPerfect. Today, she called to my attention that
one of the missing files was one she had typed on her computer about 5 days
ago, saving it to my computer's hard drive. And I can see the missing files
on my
computer. She cannot see them from hers. Neither of us has attempted
anything like "hidden" files etc. The network is encrypted, but her
computer has the key and can otherwise access my computer.

Does anyone have any ideas about what could cause this?

Thanks.

Rick Silvester