file sharing only works for some files, not all

I have an odd situation...

I've got four computers all linked together using two separate routers
- a Linksys BEFSR41 wired router, and a Linksys WRT54G wireless router.
Two of the computers are wired to the BEFSR41, one is wired to the
WRT54G, the other is on a wireless connection from the WRT54G.

The two routers are uplinked together using a crossover cable, and all
the computers are on the same subnet - the wired router is 192.168.1.1,
and is acting as the gateway and DHCP server. The wireless router is
192.168.1.2.

The problem is that I'm having issues with Windows file sharing...very
odd issues. I can see all files on all shares, that's not a problem.
The issue is that I can only copy certain files across the network, not
all files. For example: I have a share on the computer that is wired
to the WRT54G that has several MP3 files and several text and Word
documents. I can copy any of the documents across the network without
any issues...I can copy NONE of the MP3s across the network. I always
get an access denied message when attempting to do so.

To further confuse me here, I CAN copy the MP3s across the network if
they are in a subfolder of that main share...and I CAN copy all files
if I'm on the wireless computer. This led me to believe that maybe it
was an issue between the two routers, but that wouldn't explain why I
can copy some files, and also why I can access the MP3s from
subfolders.

Can someone please help me? I'm extremely confused by this...