Is the tuner meant to handle both digital and analog cable? If it only supports a certain type, that'd explain why you're getting limited channels.
Beyond that it could be any number of things. Setup, possible installation problems, etc. When I bought mine I had a lot of trouble getting it to function much at all. So the fact that you're getting at least some channels is encouraging. I was able to contact someone in support and through a series of emails we solved the problem. Maybe you should try the same.
"Don't take anything for granted, because tomorrow is not promised to any of us." ---Kirby Puckett