It is possible that your color and sync wires are in the wrong position.
Color wires start from right to left. red green blue ground. next the 3 pin plug from right to left should be ground,sync,sync. Unless all your sync's are in the 6 pin plug then they would be after the ground wire. I believe your game is neg. composite which they will be in the 3 pin connector, if the game is pos sync then they will be in the 6 pin connector.
If everything looks good then check if pin in plug is crushed and not making connection.Also check your edge connector pins. Maybe they need contact cleaner sprayed on.
Check transistor around sync plug and compare to the others and see if one is bad. some will be npn and others pnp transistors.
Try adjusting horz pot and vert pot?