Straightening a neck

Im straightening the neck of an accoustic i have but unlike the zillion other necks i have straighten in the past, this one "sticks" everytime i try to turn the allen key. It was really bad when i started tightening, but i gave it some effort and would hear a "snap" everytime i get the thing going or turning but then would stop after a half turn. It would be stiff again and the same thing...but now im closing in on a straight neck and need a little softer touch then brute force...anysuggestions to make this easier? I assume the wood has contracted up the neck or metal expanded or something...and the snap is the sound of the metal starting to twist against the wood...i know the snap isnt the neck breaking... Let me know, thanks.

Some things I might try if I were you is steaming and heating the neck. This would probably cause it to bend more though if you don't continue with what you're already doing while you are heating and steaming it. Other than that, I might set the guitar flat on its back with something laid on the body to hold it down and a light weight object placed on the top of the neck.