First of all, Kevin Thomas is a fund of knowledge. He's correct, of course, that the combination of the design of the 300H&H with the unfortunate decision to neck size these handloads is a recipe for problems. Depending on the "hotness" of the load, some type of failure to eject might easily have occurred much sooner. But, I have a question of the OP. Please clarify this part of your post for me:
"Today, nearly two-thirds of the neck sized cases did not extract when the bolt was pulled back. Subsequently, I did a full resize on the same lot and they all extract now."
Okay, extraction and ejection did not occur? So, what did you do, pound the bolt handle with a hammer, or use a cleaning rod down the bore, or what? What was the condition of the cases? Primer okay? Did you check case head expansion, any other signs of excessive pressure? Do you pay attention to brass flow and trim to length, which is important in a design having a long sloping shoulder: speaking as a person with extensive experience with a couple 220Swift's, same problem, semi rimmed, sloping shoulder, high operating pressures, etc.
Second, why did you continue to shoot these loads and deal with the difficulty of extraction? It seems to me that you should have stopped and evaluated the condition long before you accumulated two thirds of the total, however many we are talking about? Personally, my alarm bell goes off with the first (abnormal) event. I'm not judging, but you really need to pay attention to these warnings, way before you exhaust your supply.
Bottom line, get yourself a nice full length sizing die, (adjust it correctly) and sin no more. BB