Air in the system will not cause the wheel to seize up. Flushing the brake fluid won't help with a seized up wheel either.
Does the spindle (or center) get hot? If you can touch it without getting burned then the brake(s) are likely simply out of adjustment. Assuming its a drum brake system there is a start nut under the axel between the shoes than simply needs to be turned in. Depending on the manufacturer you may not even need to pull the wheel. A brake spoon or in the absence of a brake spoon a straight screwdriver will work.
If the spindle is hot but the drum is cool then change out the bearings.
I bet the brakes are simply out of adjustment otherwise the wheel probably would have come off in a very horrific manner by now.
Either way get it fixed before moving the trailer.
If you want to discuss via the phone PM me.
Good luck.
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