Are you sure there are no orphaned users?
http://technet.microsoft.com/en-us/library/ms174378.aspx
-- Detect any orphans
EXEC sp_change_users_login 'Report';
If you already created server login named 'Mary' on SQL Express, use the following command to remap.
-- Remap to same existing login
EXEC sp_change_users_login 'Auto_Fix', 'Mary';
You can use this optional syntax to create the login with a password.
-- Remap to new login
EXEC sp_change_users_login 'Auto_Fix', 'Mary', NULL, 'B3r12-3x$098f6';
GO
In a nutshell, you can use SSMS to manage that SQLEXPRESS server. Poke around and make sure the logins and users are mapped correctly. You can even test if you have the Trusted login credentials.
If this still does not work, please post back to me.