The most common issue we see is with client/staff users trying to login at the same login page used by the dashboard administrator.
Client/Staff users can only login at the white labeled domain/sub-domain that you've configured within your white label settings.
You can easily find this URL via your Users dashboard: It's shown at the bottom of the screen.
Conversely, account administrators will log in at app.agencyanalytics.com (this URL will not work for staff/client users).
If your users are going to the correct login URL but they still don't have access, ask them to type their password into the login area rather than pasting it. Sometimes copying and pasting a password can cause other characters to be added, which causes an incorrect password to be used.