If a user is changing their password at each login, it is likely they are using incorrect login credentials. Once the password is reset, they will be able to log in - but then upon logging out and back in, their credentials fail.
If you have a user that is unable to log in without resetting their password, please have them provide the following:
A screen shot of the login screen, with the username and password they're trying to log in with (passwords will not display), after clicking Login with the error message displayed:
You can then compare the username in the screen shot to the username in their profile:
If the usernames do not match, the user will need to try signing in with the username listed under their Sign In Information.