IF the refill does not stick out far enough teh barrel is too long.
First,measure it.
I know that seems simple but that is the first thing I do just to appease myself.
It could be the "cartridge" is too short,but I am hardly that lucky.
You did say cartridge in an American style pen.
From this I gather you are not talking about a ball point.
In the past when this happened to me,I dissassembled the pen and after closer inspection I either
A. forgot to mill one of the ends,or
B.
I milled both ends but the debris from milling"blended" with the edge of teh tube and gave the illusion that I had milled it to the proper depth.
I imagine there is always the possibility that the tube was too long also.
In my case I have started looking at the milled blank under an illuminated magnifying glass I got at Walmart.
It is what I use near the lathe but am going to get another to install by the drill press.
Hope this helps.