Login Scripts

I have a NT server and win 98 m/c as clients. Since each user has to have a different drive mapped to his login, I have written login scripts for each user (using commands net use f: \\servername\sharename) these have been stored as batch files in the winnt\system32\repl\import\scripts folder which has been shared as netlogon.
i find that sometimes the logon script does not run when loggin into the network. Shutting down and restarting does work sometimes. And for some users it just does not run.
The name of the bat file has been mentioned in the ‘user properties\profile\logon script name’

Why does this happen ?