hi alco ... over simplified, it is probably just tolerance stackup in the cartridge assembly ... the direct approach would be to put dykem or nail polish or paint on the cartridge that doesn't chamber .....including bullet and all of brass .... then gently try to chamber and then gently remove and see what rubs.
a less direct way would be to measure suspect areas .... start with case neck diameter of the offending loaded rounds ... maybe different brands or lots of brass .... then lengths of the brass .... then the roundness of the bullets ... even with name-brand molds it is common to see 0.003 runout ... they shoot better than they look, usually .
check that the bullets are all seating the same ... if light neck tension a sticking bullet might be seated out further .
also note that the same offending cartridges should repeat the hard chambering .... so that it is not some hangup with the rifle itself ... extractor or firing pin assy binding etc .
just some thoughts. let us know what you find ... or if it remains a mystery ...
ken