I'm reluctant to post this advice, I don't want anyone to damage their bass, but...
Imagine your bass laying on a workbench face up. Find something firm but also soft (i.e. no give, but also won't dent your neck finish) to put under the neck about halfway between the headstock and the body. Or another way to do it is to lay the bass such that the headstock is in the air to the side of the bench, and then clamp the body to the bench (with a padded clamp, don't hurt the finish). If you're trying to put more tension on the truss rod to lay the headstock back, then you can gently but firmly push down on the headstock as you simultaneously attempt to tighten the truss rod. You probably need somebody to hold the body down to the bench if you didn't clamp it. You're basically helping the neck move in the direction that you want the truss rod to take it. That reduces the friction/resistance that the truss rod sees and might let you take it a little bit farther than you otherwise could.
Just please don't be a gorilla when you do this. You need to be firm, but don't go all Schwarzenegger on it.
Also - the newer G&L truss rods can be confusing. If you get nowhere in one direction, maybe try the opposite direction and see if that helps? In fact, maybe start with this before trying the above.