KWA USP only fires double action

[maverick5770]

Hello,
I have a KWA USP compact, it has suddenly developed this quirk, it wont fire singe action, only double action.
If I insert the mag and rack the slide it will fire the first BB no problem.
It chambers the second BB as well and the hammer is cocked, but when i pull the trigger it only falls to the spot where it sits if you use the de-cocking lever.
Pull the trigger again, cycling the hammer with the trigger, and it fires perfectly, shot number four, same issue, hammer only falls to the de-cocked position. It continues like this through the whole magazine, fire, miss, fire, miss, fire, miss.

I have 3 different mags and all three of them are doing the same thing, i have contacted KWA but i thought i would ask here too,

Thanks in advance