There could be several things going on. First, is your gas block adjustable? If so, open it up all the way to help aid in cycling the subs. The gas block needs to be open all the way to allow as much gas as possible to cycle the action. A piston gas system is easier to cycle subs than direct, so it should work. Also, I would check to make sure your gas block is lined up properly, since you added a new hand rail.
Next, 185 grain subsonic ammo will be harder to cycle than a much heavier bullet, like a 208 or 220 grain sub. The dwell time on the 185 grain is less, therefore having less time for max pressure to cycle. The suppressor needs to be on the rifle when shooting the subs. The suppressor adds more back pressure to the round and helps aid in cycling.
Who's ammo is it? I don't know of a 185 grain sub load. If the ammo is loaded really slow (900-950 fps), there is less powder inside the case, which means less pressure to cycle. Some factory ammo is loaded slow, even for a subsonic load. I have spent A LOT of time and research on my subsonic 300 blk out ammo to get it to work just right in a given barrel length.