I took down the monkey gun demo today and decided to open up the malfunctioning Target Dropper box.
As mentioned in a previous post, the magnet in the drop box had mysteriously weakened so that it could barely hold a short chain of paperclips.
Upon opening the box, I discovered a neodymium magnet attached to a random point on an inner wall. I pulled it out and set it on the back of the electromagnet. I reassembled the box and gave it a try.
It worked! Still a bit weak, but more like 80% than the 1% it had been.
So it appears that the supermagnet that's used to give the electromagnet some permanence had popped off. It appears to have been glued. If I need to repair it again, I'll probably use a new supermagnet and maybe add a bit of glue. For now, I'm happy I was able to effect a repair.