Everything he said....
Magnets do wear and do go bad without wear, but at only 6 years old I suspect they are either worn or other issues exist.
Magnets are cheap to try. Trying is not the worst solution.
The grease on the brake pads sounds likely since it locks up in the air but nothing on the ground.
Check wires. Trailers are notorious for having terribly done wiring for brakes.
Check that the mechanism physically works (it should that you said it locks up in the air).
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