They won’t retract fully if the pads have worn; more piston will show the less pad material there is.
If you know that already and the pistons are sticky though. Take the pads out, squeeze the lever a couple of times go get more piston showing (be careful not to pop them out). Then use silicone lubricant to lubricate the pistons/seals. Sex lube works well and is easily acquired! The Durex stuff that comes in little bottles.
Hope recommend silicone (non oil based lubricant) for the job and supply it in their dealer kits.