Mitchycod

As i mentioned in the other thread for a new gun i'd just send it back, theres nothing in that picture that looks out of place but i suspect the problem is internal with arm-piston interface.
 
Either the ridge on the piston is worn/broken, the the arm is worn/broken, or there's just not the right angle/fit between them thats letting it ride over rather than lock into place.

It has a 45° sear, which is made of pot metal. Plus it comes in at 450ish FPS, which isn't good for that kind of locking mechanism.
Moreover, the piston is plastic, but it's not AA T-10 quality, it's made of some cheapy plastic and doesn't last very long even at 1J.
 
Unfortunately it's the theme of these VSR clones (even the TM one has 45° sears and plastic piston, but it clocks in at 0.8J and the quality/tolerances are miles ahead).
 
I'd send it back and swap it for a better gun, the A&K M24 comes with 90° steel sears and it'll last much longer, on top of giving a smoother action of the bolt.

that's your sear engagement surface, the lower part in that window is the trigger side, while the upper part is part of the assembly that holds the bolt in place.
 
when the bolt is cocked the action of holding the piston back under spring pressure should push the upper part down until it's stopped by the lower part, pulling the trigger moves the lower part out of the way allowing it to drop firing the gun.
 
the reason there's a window there is so you can visibly see when adjusting the trigger pull what's going on, eg how long or short a pull is required to trip the sear.
 
it's possible to have the overlap set so light the mildest jolt could make it trip, but it looks more than enough there.
 
i suspect your problem is more on the upper side, where the upper assembly has a hinged arm that catches on the back end of the piston.

hmm, did you get this secondhand?
 
if it's new then better off going back to the retailer.

when you pull it all the way back is there any kind of final click?
 
it sounds like the sear isn't holding the piston back, meaning it's springing forward- either you're not pulling it back far enough to engage the sear or the sear is damaged/broken.