cxna767 and cxna777 cassette decks dont work please help!?

I just bought a cxna767 and a cxna777 3cd dual cassette stereos (used) from 2 differant people this weekend. Everything works great on both of them except the cassette decks. They both have dual cassette decks and neither deck in either of the two units will work. I select the tape function and press the '< >' play button and nothing happens. The display will show which deck I have the tape in but the display counter doesn't move nor does the tape. If I select autoreverse it will cycle back and forth (the lights on the '< >' button) but nothing happens. I can't believe that 2 differant units from 2 differant people have the same problem. Am I doing something wrong