This is in the gun, not your technique. Seating primers just isn't rocket science and really doesn't require a micrometer. If you press until it stops -- it's SEATED. That's it.
Either your gun's strain screw is backed out causing too little tension on the mainspring, or the hammer is hanging up and not getting enough speed on the fall to deliver enough energy, or you need a heavier mainspring.
The screw is in the lower front of the frontstrap of the grip. (Depending on what grips your revolver wears you may have to remove the grips to see the screw.) If you put a well-fitting screw driver on it and tighten it firmly, it's tight enough. If it wasn't tight, there's your problem.
If it lights all factory primers (try several brands), it could be that the MagTech primers are much harder than normal. On the other end of the spectrum, Federal brand primers are very soft and will light off pretty consistently even in guns running light springs. If it lights off all factory ammo, S&W is probably going to say that it's within specs.
If it isn't consistant with factory ammo, S&W may take it back, replace the mainspring, and even polish up the hammer channel to help it move faster.
If it is still an issue, you could replace your factory mainspring with a Wolff "Type 1" spring and see if it improves anything. The Type 1 springs aren't heavier, but they do feel different/better than stock. Just might work better, but for $12 they could be worth a try.
-Sam
