The trigger mechanism appears to be fine. With the action out of the stock, I can push the action bar down towards the mainspring and it will catch and hold. If I try and #### it without pushing it towards the spring, it just skips over the point where it needs to compress the spring.
The cocking lever is not supposed to act upon the spring to #### the gun, it is supposed to be pushing against the piston.
From your description, it sounds like one of two possible problems:
A) The foot of the cocking lever is missing - either broken, or worn away, and therefore not engaging the piston as it is supposed to, but rather, is engaging in between the coils of the mainspring.
Can the cocking lever be lifted out of the slot easily - ie - with the gun laying upside down on the bench, can the cocking lever just be lifted out of the slot that it fits into, with no resistance?
If it can be, the cocking foot is definitely gone, and you'll need to replace the lever.
B) The rear section of the piston is split open, and the cocking foot cannot engage it, but just slips past it. A replacement piston will be needed if this is the case.
A matter now of opening it up and having a look inside.