I don't know of any commonly available .454 bullet at 230 grains. Also, most of his rounds plunk, so I don't think an oversize bullet is a likely cause.
Coyote WT,
I think it's time to haul out the Magic Marker. Cover the rounds that don't plunk with the ink. Get the case and enough of the bullet from the case mouth forward to be sure it is narrowing where you stop. Let them dry thoroughly. Try to plunk them, then gently use your fingernails to pull them out and use a magnifier to look for where they rubbed. Report back and we can give you some better idea.
Also, I let my brass mix, except for matches, and if you do, check to see if a particular headstamp is the one hanging up, and if it's different from the ones that plunk OK? I've had some foreign headstamps that were thicker than normal at the necks and actually came loaded with slightly undersized bullets (specs actually allow a couple thousandths undersized). With no normal bullet would they fit in.
Also, what are you measuring with? When it comes to bullets and cases it is often worth using a
an OD thimble micrometer that resolves tenths of a thousandth. The usual calipers deflect a little too easily, and matters of fit can hinge on fractions of a thousandth.