Bolt should lock back on the last round out, even if you are shooting the rifle one handed.

Let first make sure that the barrel was installed with barrel extension lugs correctly indexed,
Shotgun the upper open, and the center of the top and bottom lugs should be centered on the gas tube channel/ lower take down lug.


Next, insert a mag into the well with the upper still shotgunned open, and make sure that the mag follower is freely pushing the bolt catch all the way up.  Before hand, you can even check to make sure that the catch is moving freely in the receiver slot for it (just light spring tension pulling it back down).

Also to point out, make sure that the back of the follower is engaging the catch tab correctly as well.


As for gas port sizes,
16" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .063" to 078"
16" barrel, .750" dia., 8.375" from muzzle;
Port sizes from .070" to .086"


Yours is in the hunt, and if the game plan is to just run 55/62 gr ammo in the rig, then you will be fine increasing the gas port to .070".  
The reason that S&W leaves the gas port on the smaller size, when you run 69gr+ ammo through the rig, that ammo uses slower burning power, which will increase the port pressure since the spike dwell of the power burn in farther down the barrel that with 55/62gr ammo's that use a slightly faster burning powder.