I have usually found on my machines that when Gottlieb drop targets hang up and fail to drop down completely, it is because the switch blades on the sides of the bank are exerting too much pressure on the metal slider cams and thus causing too much friction.

The mechanism can get gunked up also, but I'd look at the pressure exerted by the switch blades first and if it appears to be the culprit, adjust the long switch blade away from the slider a tiny bit more.

The position of the drop target bank underneath the playfield also has a little bit of wiggle room for adjustment when installing. There is also the possibility when you reinstalled the bank, the plastic drop target may be hanging up against the sides of the hole in the playfield?