I suspect that you're sometimes making an error when you enter your user name and password.
I'm so used to typing my password that sometimes I type it too fast and accidentally transpose a couple of characters. Then I need to start over again. I've even accidentally done that a couple times before I slowed down and got it right.
Be sure you slow down and type you user name and password slowly and carefully. That's especially important since your password is not echoed to the screen, so you wouldn't notice a typo. I don't know of anything about Solus that would cause logons to sometimes work and sometimes not work, as long as you give it a correct name and password, and make sure your CapsLock is OFF.
