Why is my cd player not tracking on most cd's

The length of a particular disc could be the problem. A check-up/diagnostic couldn't/wouldn't hurt.

....or it's begging you to get a computer based system. :)

Probably you need a new lens.

If your handy and want to save some $$

1st, try cleaning the lens, slide rails, gears, then lube (with the proper lube, sparingly)

2nd, try having the lens adjusted (good local tech did mine in 5 minutes)

3rd, if all the above fail, replace the lens and you're good to go, as long as the servo(s) are in good order.

I replaced the lens (Samsung) on my CEC and had my local tech dial it in, works better than the day I bought it.