I would test the magnet with a paperclip, but I would test it during gameplay. Just trigger the mode manually when the glass is off. It's what I did. If it can't attract a paperclip and the coil is getting power then you know it's the coil. Others probably know better than I do, but I'm not sure it the globe unload test actually powers up the magnet or if it is just for alignment. I never checked.

Also, when I got my Dredd, the bolt that picks up the ball was installed upside down. The bolt head was on top and the nut made contact with the ball. I reversed it since the bolt head is concave and makes better surface contact (a ring instead of a single point) than the end of the bolt. This isn't the problem you're having but thought I would share it anyway.