The info given above is pretty much spot on. You shouldn't have to worry about field strip screw tension on a minimag body though. That generally only occurs with the aluminum bodies. They have a tendency to flex more.
Take the barrel out of the body and see if it cycles without the barrel. That will eliminate the barrel from the equation. Are you using the stock velocity adjuster or an aftermarket velocity adjuster? Sometimes the aftermarket ones are too deep and don't put enough tension on the spring pack to get a high enough pressure to fire the gun.
If you are not already using it, try putting the short gold spring on the bolt. That one will allow you to operate at the lowest pressure setting, which should get the gun cycling, even if it is a pressure or force issue.
Make sure the sear pin is fully snapped into the rail. Make sure the rail bushing is in place as well.
Except for the Automag in front, its usually the man behind the equipment that counts.