Sipsey here...This is the first Ihave heard of this. We sell tons of these things. I have seen some fit slightly tighter than others, but that is due to some inconsistency in the shaft wall thickness. I have never seen them so tight they will not push in. Are you sure there is not something inside the shaft?
If they are a used shaft, maybe there is some glue residue from old insert. On the ones I have seen that are tight, I just use a shaft cleaning tool to lightly clean the inside of the shaft.
It might be possible we got a bad batch of inserts, but I would think they all have to be the same if cut on milling machines.
I get the inserts from 3Rivers, I will check to see if they have had any complaints or bad bad inserts.
If you want to ream or change anything, I would suggest you put a field point in a drill or drill press. Use some sand paper to lightly sand the insert as the drill turns it. Better to thin down the insert than weaken the shaft wall.