[TPM001]

Sounds like something is preventing enough flow of gas into the bolt carrier. Could be several reasons:

1) NPAS problem - the valve must be able to move forward to the front of the nozzle. Check to make sure you can easily push the valve into the spring and that there is nothing impeding it from going all the way forward. When you pull the trigger, the gas pressure pushes the npas valve forward to the point where the valve blocks all gas from venting out the front of the nozzle. When this happens, the gas has no choice but to go into the rear of the nozzle and into the bolt carrier. The gas pressure pushes the bolt carrier backwards, clearing the hammer which then pulls the firing pin back thus stopping the magazine from venting. This is the cycle. It's possible that the valve spring is too stiff (doubtful) which may prevent the NPAS from going all the way forward. If that's true, you can fix it by cutting the spring down a bit. If not, then that leads us to the next potential problem:

2) Nozzle problem - when installing the NPAS valve, the rear part of the nozzle requires modification. Specifically, the tabs need to be cut down and new "vent slots" need to be cut out. The vent slots "elevates" the NPAS so that it is not sitting flush on the rear nozzle. It needs to be elevated because we need gas to be able to gather behind the valve in order for enough pressure to build to push the NPAS valve forward into its spring. Check to see that your nozzle is modded properly and that the NPAS is not sitting too far to the rear of the nozzle. A previous poster mentioned a cracked nozzle The nozzle leaks could also prevent enough gas pressure to build up for a proper cycle. But if this all checks out, then there is something that could be wrong with the bolt carrier itself:

3) Bolt Carrier problem - in order for the gun to properly cycle, there should be nothing preventing the bolt carrier from moving to the back of the receiver (at least enough to clear the hammer). Check to make sure you can easily pull back on the bolt. With the hammer "pre-cocked", the movement of the bolt carrier should be relatively smooth. The fact that, upside down, your gun cycles ok, hints to me there could be a problem with your bolt carrier. Maybe something needs to be sanded down? If there ain't nothing wrong with the NPAS, nozzle or bolt carrier, then the final potential problem could be:

4) the Magazine - could be that your mag is not functioning properly? Do you have any spare mags you can use to check? It's possible that it's simply not releasing enough gas pressure. I've had mags go weak on me which would require that you replace the valve. Also check the firing pin. Is it pushing the mag valve in all the way? When you mention that sometimes the gun cycles twice (one short and one long) leads me to believe that something is not working right within the trigger mechanism or valve. Check the mag. Check the firing pin. Check the hammer to see if it's hitting the firing pin properly.