Do you mean the adjuster in the motor cage or the external grip screw?
The adjuster normally you set for lowest noise, start tight (locked up) and work your way looser, better a lockup than chewed gears.
If its the external grip screw it could be too long, it can happen with ak's that tightening up the grip will hit the motor adjuster and overtighten it, i'd suggest some washers rather than cutting the screw just in case that's not the problem.
290 could be what the spring is capable of, i'm guessing you checked compression with the nozzle in place too?
The other area where there could be a leak is between the bucking and nozzle, you can try feeling for air (or using a bit of paper) on the feed tube to see if there's any air escaping from there.
If there is a leak, assuming its a correct length nozzle with no damage then the culprit is the hop rubber, might be worn, torn, or poorly installed. However seeing as a good next upgrade to consider is a hop rubber then maybe not too big a problem.
Personally i'd give the macaron/omega nub combo @Speedbird_666 linked a go, should get her lifting some hefty ammo no problem.
if you're absolutely sure that the air seal is good then you can look at a spring change, although 290 should be fieldable unless you play a range heavy game.