Membership provider GetUser(username,false) returning null for some of the users

Question

My user exists both in aspnet_membership and aspnet_users, but its not working with GetUser method.

MembershipUser user = membershipProvider.GetUser(model.UserName, false);

Note: This works with some of the users but not for few.

Not getting whats going wrong, unable to figure it out.

Any help will be appreciated.