User permission on server 2003

I have a user that accesses a folder that is shared on a server running server 2003 std edition. The user can access some files in the folder but cannot access others, gets a error access denied.

The user has the correct permissions on the security tab under properties on the server yet she cant access some files. On her local machine sometimes under the securities tab her name is not there. When I add it , she can access the files.

No here is the strange thing. It may work for a day , but when she reboots the following day her permissions disappear again? Any help would be greatly appreciated