dear Raff,
Places to look at:
1. The playerunit in the backbox. The contacts for the ballindicator must be clean and in good position. Best to clean these contacts and rotate the playerunit by activate the playerunit stepup mechanisme. (position 1 to 20 are for play 1 to 20, pos 21 and 22 for reset). Make sure the rotor of the playerunit rotates good.
Also look at the yellow wire.Sometimes the wire gets loose. If the indication for the ball in play is defect, the double bonus will also not work.
2. Look at the contacts that choose the number of balls in one game (3 or 5 balls). You can find it at the back of the playfield. Make sure these contact are clean and make contact.
3. Look at the BX contact .