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This is mostly to expand on comments made by others and to inject some of my own experience fixing this sort of problem over the years. Along the way I've benefited greatly from the advice of others and the knowledge gained from my own successes and mistakes.
Basically, your frozen piston is coated in baked on brake fluid and dirt to the point it can no longer slide through the seal in the caliper, at least not easily. As suggested, you can have some success freeing the piston by extending the piston as far as possible and cleaning it (always with brake fluid and using something like a toothbrush) then running the piston in and out a few dozen times. Also suggested is using the backing plate of an old pad between the piston and the rotor as a guide as to how far you can go without popping the seal. When done, remove traces of brake fluid and dirt by cleaning the piston with spray brake cleaner (designated safe for the rubber parts in disc brakes) and let dry before replacing the dust cover. The grunge usually goes quite far in on the piston and it's difficult, virtually impossible, to clean it all without removing the piston. Additionally the piston may be damaged, either scored by grit or corroded, and you often can only determine that by removing it.
When a seal is new there is virtually no brake fluid getting past it, but as the seal and piston wear or get damaged, traces of fluid do start to leak past the seal and with all that brake heat it dries on solid. Add a torn dust cover allowing dirt in and the problem quickly compounds. The problem is often most noticeable after installing new pads as the piston has to be retracted over the previously exposed and grunged up area which damages the seal further. Additionally, a worn seal even on a clean piston won't properly retract the piston after braking, the brakes will start to drag, the wheel hub will start to feel unusually hot after a good drive.
My point here is, despite all your efforts, cleaning a frozen piston without replacing the seal is just a band-aid fix. The seal is already worn to a fair degree. Running the piston in and out a few dozen times will only make it worse no matter how much external cleaning you do. Doing one piston is rarely enough, others are usually sticky if not frozen. Your brake problems will soon return, well before the new set of pads is worn out. To do a proper fix you need to overhaul the calipers. If you accidentally pop a piston you're so close to an overhaul you might as well do it. An overhaul means replacing the seals, cleaning and inspecting the caliper and pistons (replacing any scored, corroded or worn ones) and treating it to a set of new dust covers. It can be a do-it-yourself project. The cost of new seals and dust covers isn't great, especially the rears. It does take a minor bit of skill and know-how to get the pistons out and then back in without making a mess, damaging the pistons or tearing the new seals. If the seal has a tapered cross section (all that I've met do), you have to make sure it's installed facing the right way -not at all obvious the first time round. For rear ATE brakes you have to position the stepped piston faces at the correct rotation (most manuals show this). After you've done it once and likely made a few mistakes, it's all rather easy.
After having said all that and unless you enjoy the challenge of an overhaul then listen to the many people like Rob who find it simpler, even cost justifiable to go with a rebuilt caliper, especially if you can find a source like FCP Groton selling quality rebuilds at a discount price. Whether you go for a rebuilt or do an overhaul, it's normally recommended to do them in pairs as the other side is likely not far from having similar problems and with a fresh set of calipers on both sides you'll have more uniform braking. This is most important for front calipers.
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Dave -own 940's, prev 740/240/140/120 & quasi-expert only on a good day
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