Sounds like a firing pin issue to me...

was the action a known good?

did you change the triggeR?

a too deep chamber is a huge problem... but.... hmm, try neck sizing the ones that went bang and try them again...

or, as I have found to happen (don't ask)...

tear down the ones that's DIDN"T fire... pull the bullets and powder, and, AT THE RANGE try them again...

who headspaced the taylor PROBABLY had the belted mag go/nogo... and it SHOULD be okay... but it may not be...

if it IS headspaced on the shoulder, you may have over zealously set the shoulder back... doubt it, but it could be...

measure fired/unfire rounds about 1/2 way in the middle of the neck, for diameter... and try (this aint gunna be right) to measure from bottom of the case to top of shoulder and base of shoulder... the fired ones should have a DEFINATELY different shoulder, btw

jeffe