You might need a headset spacer. If the fork is too tall then the topcap is hitting the fork making it not able to tighten anymore. That or you could have stripped your starnut. If you stripped the threads from your starnut then it will screw in but not tighten down on the starnut. Go and try putting in a spacer and if that doest work then go to a bike shop and tell them you tried the spacer and that you need a new starnut.