I have the same rifle, also the conversion kit for 209 primers.

If you look at the plunger face, the part that strikes the primer, is it somewhat flat or does it have a small raised dimple in the center?

If it is somewhat flat, it's the stock plunger for #11 caps
If is has the raised dimple, it's the plunger for 209 primers

I'm just trying to establish what you currently have since the rifle was purchased used. The conversion uses a different breech plug and nipple. I also agree to try a different nipple and maybe a different breech plug(yours could be out of spec and causing you issues) but I would make sure as to what plunger system is in the rifle. The conversion uses a thunder dome breech plug which requires thunder nipples.