Any one or all of the three causes you were given are very possibly the problem. If you are having this happen with cases that were originally fired in your gun, some testing is necessary to determine which cause is your problem.

Test #1: Take some once-fired cases that were fired in your gun, and before you do anything to them, see if they will rechamber and allow you to close the bolt easily.If they are OK, then meausre the cases' O/A lenght before sizing them. If they are within specs, they don't need trimming. If they are over length, trim them. Now lubricate them and size them full-length as you did with the ones that are giving the bolt-closing problem. Now clean off the lube, and measure the cases' overall length again. Is the length within specs?? If it is, OK. If not, it is likely that the case mouth expander button is dragging so that the cases are too long again. If this is happening, trim them again. If not, you can proceed to Test #2: chamber them in the rifleagain to see if they will allow you to close the bolt on them easily. If they do not chamber easily and let you close the bolt w/o too much effort, the sizing was incomplete and the die needs to be screwed into the press a little more. Screw the die down about 1/4of a turn at a time, and retest by sizing one case at that setting, etc., etc., until the case will chamber freely, but just barely! Such a die setting will now allow you to size each case just enough, but not too much soheadspace is kept correct for your rifle. (Screwing the die all the way down onto the ram CAN and often does, set the case shoulder back TOO MUCH-a bad situation! Ruins cases too soon!! You want to set the shoulder back JUST THAT AMOUNT REQUIRED FOR EASY CHAMBERING, AND NO FURTHER!!

Test #3. Once you find that the bolt will close easily, prime the cases, add the powder charge, and seat the bullet. Test #3 has to be done at the range with the muzzle pointed in a safe direction-test the loads now, and see if th ammo will chamber freely. If it doesn't, smething you did whien seating the bullets is causing the problem. Look at the bullets on the rounds that caused problems-do you see any marks from the rifling?? Believe me, IF your bullets are touching the rifling, it will be obvious!! If so, the bullets are seated out too far, and need to be seated deeper.

Let us know what you find to be the cause!! Good luck!!