I had to do several of the pistons several times as the bottom oil ring would pop out and prevent the ring from entering the cylinder. This shouldn't take a lot of force, and if it feels stuck it probably is. I noticed it helped if I lightly tapped the edges of the ring compressor down with a rubber mallet to be perfectly lined up and even with the top of the block. This way there was no gap for the rings to pop out, and by keeping downward pressure on the ring compressor, it made it easier