1 trailer brake locks up

Incorrect shoes 4515, 4707 or Fruehauf shoes mixed up?

 

Incorrect shoes as stated above or they just cleaned the old shoes off and did not replace them. Nothing wrong with that if they weren't soaked in oil to badly. but sometimes it takes a little while for the heat to bake the glaze off so an old school trick is to run them a hair looser then the others for a few days then gradually tighten them up. Usually works unless the shoes were soaked so bad then they shoulda been replaced.
Another old school trick is to lightly sand them with an 80 grit DA or steel wire brush on an angle grinder to rough them back up. Nothing aggressive just massage the surface to restore it.

 

Might be a PITA, but I'd try to scale each position independently with a load. May be the the one locking up is lighter. That's why I asked about the air bag.

Possibly the bag was replaced with the incorrect one

 

I do not know but will share some things I have seen.
Fruehauf shoes in place of 4707 shoes. They have a curve on the anchor end of the shoe where the 4707 is straight. 4515 shoes should have a small hump where the large spring is at. 4707's do not. Do not remember which mismatches caused wheel lock up.
Budd drums on hub piloted assemblies. They most often will be off center as they do not center on the hub or the studs.

 

That's a support stand. Could keep weight off one side if it's completely broken and 1 side stays down