Are you sure that the coin mech is programmed for coin interface, and not some serial interface? (i.e. NOT enabled for NRI or MDB serial communication.)
If it's setup for serial, it is rejecting the following coins because it did not get the proper response from the game ('machine') after telling it a coin was inserted.
I can't help there with the C220, as I'm not familiar with them. (I only know the NRI G.20 and MEI Cashflow 440)