Council mechanism

I've got an old tuppeny nudger mpu4 machine I'm having a bit of bother with.

20p coins are taken in but only 70% of them are transfered into player credits.
At first I thought it might be the coins themselves and separated the ones that are credited only to find that the credited coins sometimes aren't credited by the machine.

10p & £1 coins are not a problem.

Anyone any ideas??