Authors have login problems

I have a strange problem with my WPsite, I have different people all registered as authors in the site.
They all have the same problem. Sometime the login process go on succesfull, other times they have to require a new password to login.
I searched for a reply in this forum but didn’t find it. Someone can tell me what should it be? I don’t know what to think cause this problem doesn’t happen all times…