I have replaced many caliper seals on bikes and cars and the main reason for sticky pistons can be 2 things. A corroded piston causing it to be tight or corrosion inside the caliper behind the seals, The later being the most common. If the pistons look rust free pull them out and then carefully remove the seals with a blunt pick noting the way they came out then clean the groove they were sat in. The groove is what usually gets corroded which then causes the seals to pinch tight on the piston. Reassemble and hey presto as good as new. Inspect the seals before refitting them as they can sometimes start to split.