Re: 300 Brass Stuck
Never hurts to have a few extra shell holders on hand, even of different makes. They all vary a bit, and such variations can combine with minute differences in the brass itself to bind up a case. In some situations, you'll find two pieces which are both "within spec" but will not function together, although they theoretically should. It happens. Not at all uncommon to find that in situations like this, where one shell holder won't work, and other "identical" shell holder (same make and number), will work just fine. Nature of the beast, don't let it worry you too much. If you can, take some of the cases that aren't working down to your local gunshop and try them with a different (same number, most likely) shell holder. Might also be worth trying those of different makes, too; if you're using an RCBS, try a Redding, or vice versa.
Not to belabor the obvious here, but I'll agree with MagnumManic, too; confirm that you're using the correct shell holder before trying the other fixes I've mentioned. Hey, we've all done it at one time or another!
Hope this helps,