Well Stalker's no help, man why do we tollerate that guy

.. so I seek the help of you the awsome asc community...
KSC G17, stock, plastic slide, 2 new mags... Bought as parts gun, then worked out what it would cost to replace what was missing, so it has all its parts, just put it back together.
its finally not shooting out all its gas, but its not cycling at all, no blowback, just shoots the bb and thats it.
Any ideas to the cause?