Machine doesn’t detect when the “prize” was picked up and keeps going

When a prize is picked up and dropped into the prize hole, my machine keeps playing music as if the nothing happens. It doesn’t seem to be able to detect when a prize has been successfully picked. It will continue playing the circus music, until I get the losing sound since it thinks we haven’t picked up anything.

I suspect the sensor is broken or something like that? Everything else works fine, lights, sound, controls and all. Except being able to detect a winning game.

What’s wrong and how to fix?