Why can't some of my employees login

There are a couple of reasons as to why your employees are having issues logging in to the portal, the more basic one would be that they have forgotten their password. This can be fixed by clicking on the 'Reset your password' button on the login screen. 

The other reason is a bit more obscure. If you have more than one company, or you are sending an employee from one company to the other, it is important to remember that they will need to use the correct URL for the portal, that matches the new company they are using, otherwise it will not allow them to sign in. They can check the URL in the address bar at the top of the screen.