Monaco is a late enough game that it has internals similar to a Bally game. As such, there are a few relays that will chatter away while it finds balls in various holes during the search (which happens continuously).

If you're not hearing the chatter after the 5th ball is shot, the payout will never happen.

If you are hearing the chatter, then next I would look at the search disc. In United terms, this is called the Scan disc. It needs to be clean, and the wipers that ride in the slip ring grooves may need cleaning as well.

Also check the alignment of the fingers on the scan disc, as they need to sweep a column of rivets at almost the same time in order to grab and hold.

Next, there is a search index relay (Bally term) that exists under the edge of the control unit near the Scan disc. This needs to engage and grab the gear to hold the scan disc in place during a payout. Finally, there is a replay cams index coil (again, a Bally term) to the left of the search index, that will engage and allow the portion of the control unit that pays games to turn, racking up credits.

If you have some pay but not others, it is most likely a dirty scan disc. Take a look there first and hopefully the above sheds some light on the process.

If none are paying... then there is likely a timing/ball count issue.