accessing patient portal

I cannot access my physician's patient portal. In the past, I could access the portal using my iPad and my Mac Air. Now, when I try to sign into my portal by entering my user name, the sign in page keeps circling back to the same enter user name page. I have tried setting up a new portal account several times (new user name, new password, new security questions and new activation code from the doctor's office -- which seem to be accepted by the portal) but when I try to log in, the system keeps circling back to the same enter user name page. Any suggestions?