Did you make sure each drop target switch registers when you manually drop them? You can do this with the glass off during a game, or in "switch test" mode. If one or more of them are not registering when activated your reset coil will not reset the target bank. Apologies if you've done this, but you only said they "look" good.

*edit* is this a Bally KISS or a Stern KISS? If it's a Stern try disconnecting the plug on the drop target board and re seating it.