first look at the level of the brake fluid in the reservoir, make sure its not totally full. That's one possibility if you added fluid after the old pads were partially worn. When you push the pistons back in the calipers when replacing the pads, if there's too much fluid in the reservoir you could get a hydraulic lock situation. But, if you can retract the pads with a screwdriver then this probably isn't your problem.
if the level is OK, then your next step would be to look for corrosion on the caliper pistons. That increases friction so they don't retract normally after removing brake pressure.
A third option would be that you didn't replace the parts correctly and that is causing some other type of mechanical interference, check that everything seems to be aligned correctly.