some valid users not able to login with ActiveWindowMembershipProv

[Mahavir]

For some(very less in fact) users, our forms
authentication based .NET application (using ActiveWindowMembershipProvider)
not letting them login.

When we checked we found that, the MembershipUser object returning null
for those usernames !!

Other usernames are getting validated and returning proper MembershipUser
objects.

What can be the cause and how we can resolve it?

Thanks in advance for any replies.

Regards,
Mahavir