There is a number of possibilities. If raising the action hasn't worked sheck the fretboard isn't bent. You can do this by holding the guitar and looking down the fretboard and it should remain totally straight the entire length. If not, i suggest you take it to a guitar shop. Also, if it is a bolt on neck, it may have come lose, have a look on the back and see if any bolts are loose. If they are -which they really shouldnt be- try tightening them or take it to a guitar shop
Alex New, February 2008