mapped drive connects to the parent folder

im having a problem with certain users not being connected to their mapped drives.
here's the setup:
all users used a roaming profile so that they can use any computer in the network. their homedrive and profile folders are stored on a server in a central location. each user has a mapped drive (call it Z:\) which connects the user to their homedrive.

when users log onto the computers in the building across the street (on the same network) they SOMETIMES find that their Z:\'s are not connected properly. they DO have a Z:\, but it is mapped to the folder above....(eg. rather than the Z:\ mapping to '\\server\homedrive\user1' it connects to '\\server\homedrive'......???

it's really annoying as i cant find out 'why' it is happening.

i do, however, find that logging the user off and logging back on again (not restart) connects them to their z:\ correctly.

any suggestions...:4-dontkno