Is the bolt locking all the way back on the bolt catch on single shot, or does it let go when you drop a mag? If it lets go, it's not enough gas.
Running a rifle length gas system on an 18" barrel means both gas pressure and dwell are diminished. I'd try a very wide open non-adjustable block on that rifle. And standard carbine buffer or, at max, a +1. Maybe even a lightened buffer spring.
Added:
Just a Q to clear up one possibility:
How much free space do you have between the bolt lugs and the bolt catch when you pull your charging handle all the way back?
Running a rifle length gas system on an 18" barrel means both gas pressure and dwell are diminished. I'd try a very wide open non-adjustable block on that rifle. And standard carbine buffer or, at max, a +1. Maybe even a lightened buffer spring.
Added:
Just a Q to clear up one possibility:
How much free space do you have between the bolt lugs and the bolt catch when you pull your charging handle all the way back?