Bolt sticking

[LRRPF52]

There are a number of things that can cause it:

Incorrectly timed extension or pin digging into the upper with excess torque

Bent gas tube

Gas block installed far too back, binding the gas tube flange inside poorly-machined on on-edge spec carrier keys that don't have enough bore depth

Extensions that haven't been deburred

[Wafavre2]

For the sake of accuracy any tension, binding between the gas tube and receiver and gas key will have a negative effect. Look for shinny rub marks along the tube and pay close attention to the end of the tube that seats into the Gas Key.

Will

[bj139]

I take the upper alone without the lower attached and lube it up and slide the bolt in and out to see where it might be hanging up.
Sight into the top of the upper to make sure the gas tube is centered. If not bend it over a bit.
It should be easy to move the bolt back and forth with fingers alone.