replacing rear breaks and piston will not go in deap enuf

hey guys i got this 96 xplorer 400l replacing rear brake pads and i got the piston in a little but not enuf to put new pads and to have enuf gap to fit the rotor……….. it looks like a twist piston like they have on some cars but even with pressur and twisting it .it will not go in all the way 🙁 what could be wrong here