If it is sticking, then the carrier is too tight. Shims won't help. If it was alaready too tight causing chuffs and you installed a tighter carrier, then the carrier and oring can get pushed back by the bolt stem which keeps the vent hole exposed. Any residual pressure in the chamber will leak out the front.
It could also be a tiny on-off leak.
I would try to tune the level 10. If the marker fires and then sticks, or if it chuffs when firing a shot, then go to the next larger carrier. If it still doesn't fire properly, go to the next larger one again. Get it to the point where you are using the largest carrier that doesn't leak. If you are getting any sticking at all, then you are probably using a carrier size that is too tight.
When you install a level 10 bolt after using a level 7 bolt, don't forget to increase your velocity setting. That will have an effect as well, since the level 10 requires more chamber pressure to push the bolt forward at the same shooting velocity.
Except for the Automag in front, its usually the man behind the equipment that counts.