Another thing to consider. I think the makers of these inserts expect them to be used in modern O/U's or SxS's with ejectors. It could be that ejectors snap the corresponding part on the insert quickly enough that it breaks the hull loose. Extractors will more slowly push the insert's lip, and maybe that explains some of this issue.
I think in your case, I would clean the O-ring and keep grease off of it. Maybe experiment with a thicker O-ring. The inserts I had were designed to be tapped out with a rod from the muzzle. If they don't require a stiff push to get them in, then they probably won't stay there in use.
It may also pay to polish the inside of the inserts' chambers.