This video shows you how to take the weapon apart, do this with the weapon UNLOADED.
Clean your weapon with a damp (not wet!) piece of cloth on the end of a cleaning rod. Alternatively just use a large match-end or something long and thin to feed some cloth through your barrel. Get rid of all the dust and make sure both on your magazine and weapon (where the gas goes in) there is no large chunks of dust. Make sure all of the gas 'valves' (the bits where the gas goes in and then comes out) are also clean and not blocked.
Load your weapon, fill it with gas and fire a few shots, if this doesn't work then unfortunately I have no clue.
You should clean your weapon every so often anyway.
Hope this helps you.