Gun is not in battery (ready to drop hammer on firing pin) until the rear surface of slide and rear surface of frame just below the joint between them are perfectly flush.
This is not a jam, or a failure to feed. It's a failure to go into battery.
If you push the slide forward with a strong tap of heel of your off-hand, does this resolve the failure-to-fully-chamber problem and let it fire? If it does, do you keep the disconnector tip lubed with a drop of oil every time you take gun apart for cleaning?
Are you using range pickup brass to reload? If so, you really need a chamber check gauge to see if your brass has a base bulge common with loads that are hot enough to swell the case just in front of the extractor groove. Barrel plunking probably will not find these bulged brass situations. Spend $20 or so and get a good loaded round checker gauge!
The most common problem new .45 auto loaders have is they run into brass with bulges at the base, which the sizer die in .45 auto will not remove! That is where the Lee bulge-buster kit comes in really handy. This consists of a push-thru die and a push rod which install on a spare single-stage press. Even a loaded round can be pushed through this die safely. But the best way to use it is "bulge bust" all fired brass that you know you did not shoot from your own guns, before you reload it. If you sometimes shoot almost +P level reload ammo, you may want to bust all your own fired brass, too. The bulge if not removed, sometimes means the round will not fully chamber, thus will not fire.
Second cause of failure "just short of fully chambered" is a bullet seated long enough to jam the ogive of the nose into the very short throat of the 1911. If you find the rounds that do not chamber and inspect the exposed bullet nose for rifling marks, it should tell you if that round was over-length for your gun. No part of the cylindrical SHANK of the bullet should be in front of the case mouth. Only the rounded ogive of the nose can hang out in front of the mouth.
good luck, GJ