That’s weird. I’ve got a circlip tool I’ve used with my modena engines and works well then went to change tm piston and the circlip for it got stuck towards the end of the tool. I forced it through and basically deformed the circlip so when it sits in the piston it has no tension now.
I read somewhere that sometimes using these tools the clips require some modification like slightly reducing the length by clipping the end off to make shorter
Yes it can happen. Measure your tool, the 15mm is in reality 14.5mm or so to fit inside the piston’s hole, so longer and tighter circlips “bottom out” too soon in it. If it gets stuck, don’t push it through but rather remove it from the back of the tool and install it manually with a pick or small screwdriver.
You can lightly file the clipped ends of the circlip to remove their sharpness, so they won’t damage the piston when you install them