Well, the first thing... make sure your screws are long enough. They should go well into the heel of the neck. It doens't matter how long they are just as long as you know they won't go within about a 1/4 inch of the fretboard. Try some longer screws.
Something else that my be a problem... your holes may not be drilled perfectly perpendicular to the surface of the neck heel and pocket. This causes the screws to come in at an angel... pushing on the side wall of the screw hole instead of going in straight
Try the longer screws. Once you screw the screw in until it's flush with the body of the guitar, give it a couple extra turns... this tightens the neck to the heel pocket. If the longer screws don't work. Get some doll rod and plug the holes and re-drill them. You might need help with doing that. Good luck
