Network drive disappear

Here the topo:

I open windows explorer and do not see a network drive that was there few menutes ago. I open up a dos windows and type the command net use and the missing drive is listed.

The only way to get it back in the windows explorer list is to do a net use J: /d
and re-connect it manually!

What could cause this behavior and how can I fix it for godd so it does not disappear again from the windows explorer network drive list ?