Check the resistance of the magnet - unplug the connector under the playfield near the magnet, and put your multimeter on ohms - from memory it should be around 4 ohms resistance - someone may correct me here. In any case, cehck the working one, and see if the non working one is similar.  After this, i would be checking the transistor that drives the magnet.  I had one that was out on my TOM, replaced the magnet as it was blown, then all good