why doesnt my caliper go back on with my new brake pads

you might have extended pads you purchased some calipers wont accept them find out what pads you purchased from parts store or they gave you for a heavy duty vehicle pos for 3/4 ton

IF pads are correct..(compared to old ones) and piston IS fully depressed flat with caliper surface... Make sure the caliper slide pins where the mounting bolts go through are free and will slide back and forth ! Will pads even start to go on brake rotor??