Lots of things come to mind.
Check the cases. In particular, do the once/many fired cases, that were sticky to extract after firing, chamber and extract freely after the resizing process? Check before priming or charging! If the EMPTY but resized case is sticky to insert, or difficult to close the bolt or extract, then it isn't sized properly in some manner. If everything is smooth, then seat a bullet in the EMPTY but resized case and try it out again. If it is sticky now, could be the bullets are being jammed into the lands as they are not seated properly. Did you measure where the lands are in relation to the case base, and the top of the belt? Did you check the CBTO of a reload?
As Ed says, maybe cases are not being resized properly, especially just above the belt. Read this:
larrywillis.com/answers.html I use one of his dies from time to time on my 300WM.
Could be the shoulder needs to be pushed back. Yeah, yeah, I know, it is supposed to headspace off the belt, blah, blah, blah, BUT you really should be headspacing off of the shoulder just like any other bottleneck cartridge. If the shoulder is too far forward, it will be headspacing off of it and if it is longer than the belt to shoulder or the base to shoulder, the case will be under compression as the bolt is closed.
There are others but you haven't given near enough info. Exactly what is your procedure for checking/measuring/resizing//trimming?
