Sounds like a length problem. The battery is too long (body too short) and is stopping the case of the light from making contact with the contact ring inside the head. Line some different batteries up and see if one is a tiny bit shorter than the others and give that a go.
Also, you did look down the battery tube to make sure that you don't have something stuck in the bottom that may be stopping the battery from making contact right ?
Fact is, you have 2 contact points for the negative side of the battery. The 'battery minus to case' in the bottom of the tube and the 'top of the tube to the contact ring' in the head. If the head works on other tubes, it has to be one of these points that is failing.
