I'm using the original, but will try an upgrade. That was my next thought.
The reason that I asked is because the dimensions between the different parker-like refills are not always the same. Very typically, for instance, I need to enlarge the hole in the tip slightly to allow gel refills to be used.
I would do two things to diagnose the issue. First, remove the tip and insert the cartridge into it without a spring. If it drags at all, ream out the hole slightly. Second, remove the mechanism and seat the cartridge into it. Does the cartridge easily seat fully into the mechanism, or do you have to fiddle with it to get it in? Twist the mechanism. Does it work smoothly, or does it seem to have a catch? If it isn't smoothly operating or the refill doesn't easily seat into it, swap it out with a different mechanism to see if the problem doesn't resolve.