If the pressure pads are gone, you will have to either transfer the tape to a new shell or find a Nakamichi Dragon cassette machine that lifts the tape out of the shell to play it. This is the most common form of cassette failure -- it will evntually happen to a very large proportion of all cassettes.

The Nakamichi Dragon transport is a great machine, but they're no longer made and a Dragon in good condition sells on the used market for at least $800. Often more than $1,000.

If the pressure pad is still there, but many tapes slip, the problem is probably your cassette player. Try cleaning the heads, the pinch roller and the capstan (the rotating metal shaft) with a cotton swab and some isopropyl alcohol. Or try another (cleaned) player.