If the coins are falling into the tray and you don't get a error 21 code then your comparator is not sensing a valid coin.
Assuming you have a valid coin placed between the two sense coils on the comparator the next thing to try is to turn that small sensitivity control
in the lower front corner fully counterclockwise. Don't turn it towards the plus sign turn it all the way the other way.
If that doesn't work close the door and place your ear on the front of the machine near the coin comparator and listen for some buzzing or clacking
if you hear any then the main capacitor inside the comparator could be failing.