May you share your LDAP settings here. What kind of LDAP server is used?
Also what do you type in to the user field of the login form?
For AD sAMAccounName is used, so you have to use exact the value of this attribute.
If you use the option ‘Create users only on login’ the user must logon once before it will be synced.
We use checkmk LDAP Sync and auth for almost 2000 Users without any issues.