Bart is right, but incase you don't want to search through all of that, consider this first:
1] Check the gas key. Find yourself a perfect fitting (this means perfect fit, not close with wobble and play) allen head screw driver, and check the screws on the gas key. Try tightening them. If they move at all, they were loose and that was most likely your problem. Consider getting them properly tightened and staked down.
2] Check your gas rings. They shouldn't be bent or damaged. Also, they shouldn't be worn. When fitting the bolt into the carrier, the fit should be snug as the rings slide into the tube. Also, make sure the rings haven't "lined up"...some people think this doesn't matter, but for the sake of paranoia, see if the gaps in all 3 rings aren't lined up to create a gas leak through their gaps.
3] Check the gas tube at the front sight base. Is it loose? Did they install the gas tube roll pin? (you'd be surprised)
4] Remove the Bolt from the carrier. Take the charging handle out of the upper. With the bolt and charging handle out of the picture, slide the carrier into the upper. When it gets close to closing, about 1/2" or so, go slow and see if you can feel any gas tube drag. This would indicate that your gas tube is dragging on the carrier's gas key. The drag doesn't influence the cycle, but if it is bad enough, it *might* have an effect on the flow of pressure. It's a long shot, but it is possible.
5] Check to see if there is excessive carbon deposits on the firing pin and the face of the buffer. Believe it or not, there have been AR's made where the rear of the bolt was machined too small, or the hole in the carrier machined a little too big. This causes gas leaks.
The only other thing I can think of, is that the gas hole drilled into the barrel is too small. This happens sometimes. Also, check to see if anything is dragging in the rifle. Cycle it manually and try and see if something is holding up the show....
AH! before I forget...sometimes you need to shoot an AR a good 100 rounds so that carbon fouling "seals" up the tiny gaps in the front sight base/gas tube area and throughout. It might make a difference if your rifle is very pressure sensitive, but I doubt it.
#1 and #2 are probably your issues, if they are not, then the others are a stretch, but possible and you should send it back.
Good luck.