The pistons are hard to put in and out aren't they
I had bought a kit to redo the seals on my calipers when I was painting them because the first one I took off looked like it had a bad seal. It turned out that some gunk had gotten down in it and made it look "puffy" if that makes sense. I took it off and it was fine, and (then later after I hard time getting the pistons out and ran out of time) I cleaned it and put it back on. It was quite difficult to get it to "pop" back into the groove. You could try to rotate the seals and if they don't rotate they are probably seated in the grooves.