The problem is that the wall half of the magnet has come off the wall, leaving a hole in the wall. I tried to plug the hole in the wall with DryDex Sparkling & Nail Hole Filler, but it was not solid enough to hold the magnet's nails after drying overnight. What do you recommend?
Usually something like this:
Enlarge the hole to something like 4 x 6 inches.
Cut a scrap of drywall about 5.5 x 8.
Drill a couple of holes in the center of the patch.
Tie a loop of cord through the pair of holes. Leave enough slack to get a good grip.
Maneuver the patch through the hole, rotate it to cover the hole, then secure with dry wall screws
Cut a second piece of drywall to fit the hole. Screw it to the back patch.
Drywall mud and tape the seam.
Drill holes for drywall anchors, or use the plastic screw-in anchors.
Use appropriate screws to mount through the magnet plate into the anchors. ...