Site member login does not always work and I don't know why?

ok so I have a login for clients to set up their own password and enter the portfolio section of the site once I have accepted them as site members. Some go through he login procedure and it works perfectly. However, there are other clients that send a site member request which I accept and then for some reason unknown to me their login details do not work… I am new to this and and wondered if I have a box somewhere wrongly ticked. Or what is causing this to happen. If anybody can help that would be much appreciated especially in a time where every client counts…thank you so much