Magnetized balls? Try turning the machine off and see what that does

Did you turn off the machine when it holds the ball as butch2099 suggested? Try a new set of balls.

Just a possibility as mine had this issue with DILE, my game had some sort of mylar over the magnet from the factory? Over time it was knarled up and caused this. Used a knife to remove it and problem solved.

Feel to see if the magnet core is flush with the playfield.

Bubbling or cut mylar or glue residue over the magnet core. A pencil eraser will clean it off as well if it’s glue or residue. Touch it and or shine a light under there and you will see what specifically is going on.

If the machine is off & ball is still stuck that means it's a physical problem. Ball is magnetized or physically sticking on the mylar like others suggested. I would try to recreate with the glass off and see if you can figure out which one it is.

Also double-check that the magnet core is at the proper height - that is, flush with the playfield. If not, it's easy to adjust: loosen the BIG nut, screwdriver the coil up or down, tighten the BIG nut.

One of the first things I did with my DI was use an X-Acto knife to cut out the mylar where it was over the cores on the Theater and Drone magnets. It's easier if you lower the core down first.

While you're there, make sure the top surface of the core doesn't have any adhesive residue, and is _flat_ - a pitted core will screw up the throws like you wouldn't believe. If necessary, remove the core and clean it, or grind it flat.

Any chance the magnet core itself has become magnetized?