Once in a while, we see complaints like this one...and usually, the root-cause of the problem is that their Web Browser is remembering/autofilling a stale password when they visit the login page. 

When they see the error message that "Your Password is Invalid", they click the Request New Password link.  That gives them a one-time login link, and access to the site.   However, this does not reset their Web Browser's (stale) password.   So the next time they come back to the site to login, the process repeats.

Without knowing what Web Browser they are using, it is hard to provide them with the most-specific instructions to fix this.  But if you reset their password and send the new password to them....when they go to login, the Web Browser should ask them if the new password should be "remembered" instead of the old password.  Encourage them to do that.   =]