There have been problems with the co2 cartridge not being pierced properly, this would show when two shots are fired in quick succession and the second is low power, but two shots with a significant delay between are both ok. The gas only comes out slowly if not properly pierced.

Are you tightening up the co2 cartridge screw fully before closing the lever to pierce it?
The correct way to install a co2 cartridge is to open the lever fully, wind down the co2 screw, insert the cartridge, wind the screw back up until it presses the cartridge against the seal tight enough that it can just be turned by hand but doesn't rattle, whilst making sure the lever is still fully open. Replace the grip and smartly close the lever to pierce the cartridge.
You could also try a different make of co2 cartridge.

If it still won't work take it back to the shop.