Hey all, I've had this S&W M&P15 Sport for about three years, and have never had a problem with it in any way. This has the standard A2 front sight/gas block on it, and a few days ago I decided I would like to take that off and put a low-profile block on so I could use a 12" free float forearm. I had no problems getting the A2 off or the new parts on, but when I went to test fire it, it would fire, and not push the bolt back. So I'm assuming its not getting enough gas or any at all to push the bolt back.
I used the original S&W barrel, and cleaned up and reused the original gas tube, but used a steel low-pro gas block, USA made, but not name brand. The block looks fine and the gas port hole and block hole line up. Its been bothering me that I cannot figure this out... Any help?
I've built several AR"s over the last year, and have had this problem on another build, which I ended up changing barrels for and It worked flawlessly... But, this is the barrel that was on the gun before, and it worked fine. I had figured before it was gas port size on the mil-spec barrel, but with the S&W barrel its got me wondering again.
Any insight would be greatly appreciated. Thanks!    

How do you know the ports are lined up correctly?  Did you measure?  Or did you just jam the block up against the barrel shoulder and call it a day?  This is the number one mistake made.  Measure the center of the port from the shoulder and from the rear of the gas block, subtract, and space the GB off the shoulder by that amount.  If you did do the previous, then either you put the tube in the block upside down or the gas block on backwards (if it is the type open on both ends).  It has happened a lot either way.

... Yes I measured the holes, they line up. The gas block doesn't go on backwards fyi, and the gas tube is right side up. I'm not a beginner. Still, do you have any actual insight as to what the problem is instead of operator error?

As my departed brother would say, measure twice and screw once.  It only take a faction to be off to have a bolt gun.

Gas tube roll pin through the hole in the tube and not accidentally in front of tube all together? Sounds stupid but its a commonly made oops that's easily overlooked. If this happened the holes between gas block and tube will not line up effectively making a single shot weapon for you.

Thanks for the input, but I'm finding it to be more of an issue with gas escaping... I tried a different gas block, a low profile aluminum by Veriforce Tactical that I had laying around. Now, the holes with the tube and the block line up exactly, and the hole in the block sits over the port in the barrel. My problem is now, that no matter how much I tighten that block down, gas is escaping between the barrel and the block. I fired a few rounds thru it to test it out, and the first fired, bolt came back but did not feed another cartridge. After that it did not have the power to come back at all. I then took  the block off again, and could plainly see the gas marks around the port hole, where the gas escaped... I'd post a pic but I can't figure out how...
Anyways, is there a method or product that I can use to seal the space between the barrel and block? Being a slim profile free float tube, I have to install the block after I install the rail... Easy project turned into something difficult... Any ideas?

A clamp on style GB might seal better than the set screw style.

Yes, take it to a competent gun smith.

sounds like something is wonky with the machined area of the gas block seat, or the set screw for the gas block isn't getting a good grip on the barrel to hold it in place.  is this a set screw type of gas block,or something else?