Could it be that the the case is coming off the extractor too soon, or a weakspring? I have a Bushmaster Patrolmans carbine that was doing that, replaced the extractor spring, and buffer, and it hasn't jammed since. Mine was dropping the spent case in the reciever, and picking the next cartridge up from the magazine probably 3-4 times every thirty rounds. Give it a shot, extractor springs are cheap!
Really no such thing as "weak ammo" with the AR. That's a shotgun phenomenon, but even then it not "weak" just designed for a specific purpose and won't function well in a semi-auto shotgun designed for a conflicting purpose (light bird loads in a gun designed for heavy slug or defense loads). ARs are gas-operated, so as long as there's a good bullet seal in the barrel, the gun should cycle with ANY 5.56 or .223 ammo out there.
Gas port issues are something of an anachronism nowadays. back when barrels were being cut down from 20", the gas ports had to be opened accordingly, which led to trouble. Nowadays factory barrels are easily had in a variety of lengths and the gas ports are dead-on correct for the barrel length. As long as it's a factory barrel (by just about ANY factory), poor cycling can be blamed on other things.
That being said, what you are describing is a classic Failure To Eject, usually caused by a weak extractor spring. Replace the extractor spring making sure you have the correct plastic "plug" in it, then use an "O" ring or D-Fender for good measure. You should never have this problem again for many thousands of rounds. Carbines have much higher gas pressures than do rifles and accelerated wear of the extractor and extractor spring are the first places you'll see it. Carbines unlock faster and more violently than rifles, causing the bolt to rotate harder, causing the extractor to pivot with greater force, causing the extractor spring to compress more, causing it to weaken faster. There.
However, if the problem persists after a new extrator spring, the extractor itself may be the culprit. It might be reacting to a particular case idiosyncracy with magtech ammo, but I doubt it. Look at the extractor claw for anything unusual, strange wear, burrs, etc. Try omparing it to the functioning bolt.
90% sure its the spring. 10% on the extractor, leaving no room for it to be anything else because it isn't anything else. I guarantee it. Both of these are easy, very inexpensive fixes and it's a good idea of have spares available anyway.