If you are trying to get a really fat o ring in there, it may not fit.
Our O rings need to be quite thin, but they APPEAR to be too big and loose on the plunger. I just put the O ring on the plunger, lube it up, offer up the plunger to the cylinder at a slight angle so most of the piston and O ring are in the hole, then poke the rest of the O ring dangly bits in as I feed in the plunger.
The O ring should dangle off the plunger so loose that there may be 5mm of space at the bottom when the plunger is held horizontal.
Trying to use a fat O ring that looks like the O ring fits the plunger perfectly… is wrong. The green O ring should be about the same thickness as the original black one… only the diameter should be slightly larger
Sounds to me like you got too fat an O ring