take the caliper out and clean the sliders (a brass or steel wire brush for your drill is probably best). Put some hi-temp grease on the sliders afterwards, but not too much so it won't drip on the disk or pads. install the caliper back w/o the pads and move it back and forth to make sure is not getting wedged, if it is locking somehow try and figure out what it is. You might want to push back the pistons but not too much. Install everything back and go for a drive, lift the wheel up and make sure it spins, if it's not you're in for a caliper rebuild - you'll have to take the pistons out and replace all the rubber parts and also clean the cylinders so there is no oxidation that would block the pistons from moving back and forth. Good luck.
PS: not sure how your calipers are built, some won't let you take the booth off the piston unless it is in fully, you could try and spray wd40 for example around the piston, push the pinstons all the way in, then back out and do this a few times - it could save you from rebuilding the calipers (cleaning/replacing rubber rings and gaskets/booths) but it could also result in leaking so keep that in mind when you do it. Also, when you push the pistons in the brake fluid will overflow from your brake reservoir, so you would probably want to take some out before that. Use some hi-temp grease on the pistons as well as it would prevent them from seizing (this could be your problem).
