Mp40 Marushin Gas Problem

Hi !

I'm new here I'm an administrator at http://www.ww2airsoft.fr

I can't find any answer to my problem so I think that maybe our British neighbors could have the solution.

So here it is :

I recently bought a Mp40 Marushin with 7 magazines (yes it cost an arm and a leg).
But i can't use it correctly.

The gun only works correctly with Hf134A gas and ONLY when the magazine is half empty.
When full - winter gas, summer gas, H134A gas, the gun doesn't work properly I can only fire one or two bbs and i need to reload the gun in order to fire again ... only two bbs.

Is the Marui 134a gas different from Hf134a ?
Or is there another great and wonderful gas that I don't know about?

Thank you
ps : great forum that you have here.