When a coin is inserted and put in play, the coin detection pin senses that a coin is played, the coin advance lever pushes the coin down and pushes the previously played coin down from the view area in to the coin tube, jack pot or cash box, the coin is held there by a small lever with slight spring tension just enough to keep that coin in the view position, there is a second check detection pin that senses wether a coin or check was used to play the slot. You will need to take the goose neck off and apart, most likely that lever is gummed up or that spring may be broken or missing, that spring is very small be care full not to loose it, not easy to source one.
This link is to a sky scraper I restored, full tear down and scroll down a ways you will see the goose neck completely apart and restored to give you an idea of what you need to look for.
https://photos.app.goo.gl/QXvztvDAk9zqKz8m1