Have you done anything to the inner barrel and the hop unit?
The Aug upper uses the spring tension of the hop unit to lock the upper to the lower tightly. Since you are claiming movement on the upper it is almost certainly the hop that is the problem.
You can not use a shimmed barrel in an Aug at all. It must be free to move forwards and backwards in the upper on the spring. Likewise, it is possible to have screws that hold the hop unit in place that are a fraction too long. Same deal as the shimmed inner barrel, it stops the assembly moving on the spring.
The brass collar ring can also reduce the distance the hop can move back if you installed it on the end of the hop and it is visible in the screw slides at the front of the hop unit. I cut my buckings and put the ring forward of the C clip to stop this happening.
Upgrade barrels that are tight in the upper are also a no-no for the Aug. If the hop isn't sat as far back as possible under it's own spring tension, and it doesn't return to that position every time then the problem is there.
The hop distance is set by the plastic spacer at the back of the hop unit. Provided you have the little black spacer in place the magwell/mag, and hop will all line up. The spacer is the key to the whole system and is what allows the Aug to line up correctly. If you have lost that little locking plastic part then buy a spare hop unit to get another. Without it the hop will sit closer to the gearbox, close the mag feed, Give you an over length air nozzle and a whole host of misfeeds. It's an important insignificant part.
Adjusting the locking pin is exactly the wrong thing to do. It will put pressure on the front of the magwell and crack it. And it won't sort out the real problems. The upper might be tighter, but the hop unit will still fail to sit correctly against the gearbox.
The correct fix if all of the above is right is to remove the hop and stretch the hop spring. It'll give you more locking force without adding strain to the inner plastic parts. You should be able to feel the hop engage and the spring tension when you slide the upper onto the lower. If you can't feel any tension then you have either something set wrong, or you are missing a part.