Put them in the freezer for a couple of days.
Also, be sure the brass insulating washer is installed between the coil and the frame.

Hot or cold works. In the freezer for a few days will do it. I don't think I've ever come across one that was magnetized though. They usually stick when the coil is not magnetically insulated from it's frame.

There's usually a brass washer between the coil's core and the frame. Later Gottlieb's had plastic molded around the end of the core so that a washer was not needed. I'm not sure how it's done on "Blue Chip". I owned one once, but sold it. As "homepin" states above, there's brass in the coil stops, and the coil stop itself has a nylon bushing and a nylon insert nut to keep it insulated. That's on coils with plungers such as steppers, pop bumpers, flippers, etc.

Again, there should probably be a brass washer between the core of the relay coil (hole where the screw goes in) and the relay frame. The screw that holds the coil in the frame should be brass as well.

A propane torch would surely be hot enough, but I think something else is the issue here.

Check some of the other relays that aren't sticking to see if they have a brass screw and a brass washer. The washer does not go under the screw head, but it goes between the end of the coil where the screw goes in, and the steel relay frame.