in the form of Winchester 45 Long Colt brass.

The combination of minimal rim and groove and thick lips on the tool make it very hard the prime the cases. Most of the time the cases just fly out of the holder.

Try another shell holder that is a bit tighter.
I remember having to do the same but I can't remember which one I went to for the starline brass vs the older Winchester.
Frank

Isn't the RCBS Universal a spring loaded shell holder that adapts to all rims? If so it doesn’t take regular shell holders.

I have a little trouble with Colt cases too. Although not as bad as your problem, it's the only case design I have any trouble with. Call RCBS and tell them what is going on, they'll treat you right.

Had a similar problem with 338 Lapua Mag cases.
I resolved it by dis-assembling the unit, I think it was just a couple of small screws. Carefully cleaned up everything and I used a Dremel wire brush to polish up the bottoms of the holders.
Seemed to be a fix for mine.

Gary

Try get a later manifactured shell holder. The older onces had very tight bottoms. The newer one's has been opened up to fit the new hand prime models. Had similar problems with a Hornady Hand Prime. It had refused to work with some Lee shell holder.