Gun expelling all the gas on a single shot - Troubleshoots

BBs stuck in chamber 

Lock bolt to the rear of the gun and inspect the chamber and area around bucking to ensure
there is no bbs that are stuck. If capable, remove bolt and inspect the front face of the nozzle and bolt carrier group. If a bb is stuck in the chamber, that will cause the nozzle and bolt to not properly seat and cause all the gas to expel when the gun is fired. 

Broken nozzle/main seal

Nozzle inspection is very simple. Easiest way to accomplish that is removing the bolt from the gun and visually inspecting for cracks or deformations. Any crack is going to have pressure escape and cause the magazine to completely gas out. Deformations can cause the nozzle to bind in the bolt and create drag. If the nozzle cannot reciprocate with the bolt, that will cause gas to escape as well. 

For checking the main seal, removing the bolt is going to be the easiest way as well. Once the bolt is removed, pull the nozzle completely forward and there is going to be either a o-ring around the nozzle itself, or the nozzle is going to slide over a piston cup style o-ring. If they are dry or damaged, that will cause a compression loss and gas will expel.

Nozzle not sitting on magazine gasket properly

Tricky to inspect and repair. Easiest method is just giving the gasket on top of the magazine a visual inspection. If it looks worn out or, commonly, pieces are missing from it, that is going to create a gap from the gasket to the opening on the bottom of the nozzle and be a compression loss.

If the gasket seems okay, and you are having the same issues, then I would remove the upper receiver and insert the magazine in the lower receiver of the gun. Then using just the bolt from the gun, place that on top of the magazine to visually see if the gasket and nozzle are making a good seal. 

Magazine is too cold 

Gas Blowbacks need to have a cooldown time between fills of a magazine. If the magazines or gun is too cold, that can cause the system to gas out from not enough pressure being generated in the magazine. Easiest fix is warming the magazines before use. Simple trick is leaving magazine in pockets and letting body heat warm the magazines. Dangerous, but more effective, is going to be using a heat gun or propane torch for a few seconds. 

Internal sear issue

Remove the slide and ensure all the trigger and hammer sears are moving freely. If a sear is catching the slide and preventing it from fully closing, that will create a issue with the nozzle and seal. It will also not let the gun be fired so you might experience a loose trigger. This is common with gun that have a fire selector.