There can be a ton of reasons why this happens. Doing the mist ball test will sometimes tell you whats up. It could be the spriraled wire has a bad connection, opto weakness, or in many cases the board itself. There is small inductor/choke that can fall off the board due to age and vibration. Do yourself a favor and replace the mist opto board with a new one from homepin. You will at least get new components and a status light visible from the coin door.