I think there are different transition brackets or funnels? for quarters and bigger coins, if they get mixed up sometimes the quarter can slide past the switch without triggering or the bigger coin gets jammed up before it hits the switch.At least it is like that with dutch SS Bally coindoors.
You also want to adjust the switch to make a long sweep, i always try to make the switchclick arm travel or duration? as long as it can during the coinpass.Watch closely what happens when you insert the coin.Also test several coins with the coin door closed, The different angle might change coin path.
This is especially neccesary when you use coin comparators, where a coin is almost making a freefall to the coinswitch because of the electronic testing of the coin and not having it slowed down by a magnet as used in mechanical coin acceptors.
You might want to check if you have the correct switch for the coin you are using, there are differences in weight.
Correct coin handling is very important for operating pins in my opinion for it is a very bad experience when people put in a coin and nothing happens.It can be someones first encounter with a pinball machine and it might be the last because of that.
You really want to minimise that., i want people to have a good time on my machines and not getting a credit for your money is a very bad start.
Btw, if you use the close loop bookkeeping system, which i have, you can tell exactly how many coins are dropped without credits. Are you doing that or are the coinboxes emptied by the staff?