Rowe box, Model RI-1, help needed

It's a 120 sel and has this problem: any selections made with these
following letters, c,d,f,g,l,m, ("m" being the last letter), will register
with the reciever, start the scan motor but 'hammers' just shy of the
appropriate pin! Aarrgghh!!! The other letter/number sequences are working
flawlessly, ie: A1, J5 etc. I've dismantled the pin pushing assembly from
the mech to thoroughly clean and inspect. There is a refference mark on the
pc board, A1 I believe. It corresponds with the right pin as per #1. I'm
assuming the wiper behind this board is the letters registration. Sorry
about the terminology here, have no manual right now but didn't intend on
buying one for this entry level juke.
Cheers, Steve.