I'm a pretty active reloader but no gunsmith! (As you'll probably be able to tell with my description of the problem)
I did some reloading for a friend with a 300 rum bolt with the J lock. All my loads were checked for headspace with the stoney point gauge and all had .003" to spare.
We went to the range and fed them to the gun one at a time. About every 5 shells the bolt wouldn't close. It was kind of like the same problem you feel with a ruger if you don't feed them out of the magazine. It's kind of like they were'nt seating into the bolt face. Once the shell was seated in the chamber the bolt would not "pick it up again"....we didn't have a cleaning rod along to tap the shell out...we'd stand the gun up and lightly pound the recoil pad on the shooting bench and it would fall out. Then the same shell might go right back into the chamber normally.
Any ideas?? I'm wondering if there is something we need to look for with the extractor or bolt face?? I know the gun hasn't had a good cleaning for a while.
Any ideas for a dummy to look for would be appreciated.

Are you full length sizing, Patrial Sizing, or neck sizing?

If the round will chamber one way and not the other it may be an out of round chamber.

Or the bolt is not concentric with the chamber. I have a 7mm-08 that does something similar if I neck size. I occasionaly find a cartridge that won't chamber. If I rotate the cartridge 180 degrees it will chamber.

Let's see, if the bolt sometimes fails to snap over the case head, and, occasionally it fails to pick up and extract a case, I would look at the extractor. Get under a good light, pull the bolt and scrub the bolt face good to remove brass shavings and junk. Using a tooth pick, see how much movement is in the extractor (side to side). There shouldn't be any slop. The extractor should be tucked neatly up under the lip. If it's sloppy and out of whack, it's easy to replace.

Boy...help comes quick here sometimes!
I'm FL sizing.
I'm not sure if he had trouble before with factory fresh ammo or just that day with the reloads. These were only reloaded 1 time.

If the bolt is open and we slide the case all the way into the chamber shouldn't we be able to close the bolt and have it snap over the rim?
(It sometimes doesn't) Then after getting it out the same cartridge will go in just fine.

I also remembered a push feed Model 70 a friend has. It feeds factory ammo fine. I was trying to get him started reloading and we ran into the same problem. We were full length resizing. His rifle definitely has a chamber that is not concentric with the bolt face. On some cases the edge of the case recess on the bolt face would scrape brass off of the rim on one side. Turn the case and it would be fine.

As you probably know a cartridge case will often not expand evenly when fired. One side will bulge more than the other so the rim will be eccentric to the case body. Resizing will not normally remove all the eccentricity. Turned one way it will cancel any bolt face to chamber eccentricity and turned the other way the case and chamber eccentricities add up and the bolt won't close.

Since it doesn't seem to have had this problem before, I would clean the bolt face really well.

The other thing you can do is blacken the rims and 1/4" or so with a magic marker and see if you can see where it is binding.

Was this once fired brass from this gun, new brass, or once fired from something else?

I had the same problem once with a Remmie I had "plated". I cleaned up the bolt face and the problem went away.