OzFclass

I recently have had a few miss fires with new Palma brass and CCI 450 primers and on taking the offending cases home and pushing the primers home hard with a foster press they then ignited.

Yep I think that issue is cause for more misfires than it's given credit.
I've recently noticed with both BR4s an S&Bs that the spider is proud of the primer cup. I can't say for sure if that means the spider isn't secure so i just make sure the primer is fully home in the primer pocket so it doesn't matter.

Hello All
The anvil(what you call the spider), has to sit proud of the cup to stop premature ignition. When seating a primer it requires "feel" to know when the anvil is seated inside the cup and is contacting the medium at the bottom of the primer cup. I use a single "shot" primer seater, this gives you feel when seating the primer.
There been many articles written regarding primers, one German Salazar has an excellent article, plus Manly others.