Slider pins are probably siezed. Remove the caliper and try to move it around on the rubber covered sliders. If you cannot move the caliper body
freely and
easily of the pins, then they are suspect and either need to be cleaned out or be pressed and pulled until they move again (with penetrant fluid followed by silicone lube). I have pins that needed a ball joint C-Clamp to push them and a large mallet to get them back out, rinse, repeat about 15 times! Now people know why I was trying to source brakes, of which I just received the rears from 89mirageman.
EDIT I almost forgot that the caliper piston may be seizing in the bore, so try and compress them (front only are direct compression, the rear are twist in via pliers or a proper tool) and see if they will compress with proper feel in the movement of compression. Then get them pumped further out again via the brake pedal (don't pump too far out!) and repeat just to make sure things are good.