Did you check it before you bought it? I'd be looking for a refund before trying to sort it.
Regarding sorting it--I'm guessing it is a bit like a more complicated Meteor set up like the Supersport is?
I'd be trying it without a mainspring-----just the piston, cylinder and trigger set up and try to see what is going wrong. Try adjusting the allen screw both ways----------usually on Meteors there is not actually that much movement that does a lot but you can wind it in so the thing wont cock and hold or the other way so it wont cock at all.
If you cannot get it to cock and hold with adjustment then I'd look at the sear and see if you can get one to try (Will a Meteor one fit?).
I have heard of Meteor pistons wearing through where the sear catches and people seem to reckon the pistons are still ok to use?
BUT---if you have bought it like that I'd ask for a full refund or a partial one if you intend to keep the gun and have a go. I wouldn't sell on a gun in that condition unless the buyer was fully aware of it's conditon.
Good luck with it.