I built my first AR-15 today am I am having trouble. When fired the rifle will not chamber a new round because the bolt does not seem to come far enough back to "get behind" the next round and strip it from the mag. I then have to pull back the charging handle all the way until the bolt catches and let the handle slide forward. I then press the bolt release and the bolt slams home, strips a round and its good to fire. I tried 3 different mags, no change. I have searched the charging handle and bolt for wear or "story marks" and find nothing. Bolt release seems fine. This is a 20 inch barrel with a rifle length gas system. The lower and stock are from a 16 inch barrel with a mid legnth gas system. I don't know the weight of the buffer tube weight. Any advice would be very welcome.

what ammo and what mags. will the bolt lock back on an empty mag?

Insufficient gas?  Check that your gas block is aligned, gas tube installed correctly etc.

ETA: Take a straw and blow into the gas tube, you should hear/feel air coming out the barrel.

Did you swap buffers?

Maybe rifle buffer in carbine tube

Short stroking:  here is my recent experience on a new lightweight build.  Aero upper, McKay Industries lower, solid quality lower parts, to include Geissele trigger.  Bolt carrier group from X, stock/buffer/buffer tube from Y.  You would easily recognize X and Y, not bargain basement.  Ballistic Advantage Hanson barrel, which I first suspected as the culprit.

Solution:  I thoroughly cleaned everything.  I replaced X BCG with a new Toolcraft I had laying around, and replaced buffer and spring with one from known Colt factory lower.  200 rounds later, not a single failure with IMI M193 and old Colt 20-rounders.  Lubed with Mobil 0-20 synthetic.

I realize this is not following the scientific method, but I just wanted to shoot the rifle.  Later, I'll swap parts to find the offender.  I noted buffer from Y had no marking on it; Colt buffer clearly marked "H". Buffer springs had same number of coils, but one from Y was over two inches longer.  Go figure.