Hello there!
Textbooks can take between 10-14 days to arrive in mail. If you are proceeding through your courses rather quickly, the shipments may not be able to keep up. Consequently you may get behind while waiting. If you don't want to get too far behind, you can call Student CARE Forum 1(888)427-1000 and they should be able to open up the next couple of courses for you to work on while you are waiting.
I am sorry for your inconvenience, but I do hope that this helps. Best of luck!