You could have some bad powertube carrier orings. If these have a less than perfect surface then they require a bit more tension to seal properly. That causes you to use a carrier size that is too tight for your setup and it can cause sticking issues. Using a size 0 carrier right out of the box is pretty rare.
You won't need shims. Shims are only needed in the rare event that you have a mag rail that is out of spec such that the sear causes the bolt to sit too far back while in the ready postition. This could allow the bolt to move forward when the sear releases it, but not far enough to clear the vent hole. If this happens, any obstructions at the bolt face will cause the bolt to stop but it won't release air to be able to reset. Buy installing shims, you would push the carrier farther back so that the bolt wouldn't need to move as far to expose the vent hole. In a normal setup, the vent hole is so close to the oring sealing surface that any forward movement of the bolt will cause it to be exposed to vent air and reset.
"False leaks" are real leaks but not ones that are caused by carrier issues. We call them false leaks because they don't require changing anything that seals air. You only need to remove shims to stop the leak. These false leaks are caused when installed shims push the carrier too far back so that the bolt stem vent hole is permanently exposed or right on the edge of the sealing edge of the oring. Quite often that leak can be stopped by going to a tighter carrier size which flattens out the inner edge of the sealing oring such that it covers the vent hole. So, the leak stops but the carrier and oring are tighter than they should be and you end up with random bolt stick issues.
Have you tried a different tank? Yours might be 750, but have you actually checked the pressure with an accurate gauge? Level 10 mags act funny if the input pressure is too low.
The fact that you had issues where your mag leaked but pushing on the face of the bolt caused it to stop or reduce is an indication that the vent hole is exposed. If no shims are installed, it could be a valve or bolt spacing issue. Is your rail bushing in place?
Except for the Automag in front, its usually the man behind the equipment that counts.