If I'm reading you correctly, you're saying that when you hold the tank upright (with the stopper at the bottom) that the clunk doesn't go down toward the stopper. If this is indeed the case, you have nothing to worry about. It's not supposed to! If you are in a steep dive, negative "G's" will keep the fuel toward the back of the tank where the fuel is.

Not only do my tank clunks NOT go toward the stoppers, but I usually install a piece of semi-hard plastic tubing (supplied with some tanks) between the stopper and the clunk which is connected TO the stopper and clunk by two short pieces of fuel line. This is intended to PREVENT the clunk from being slung forward (toward the stopper) in the event of an abrupt stop (as in: a nose-over on landing.

This is exactly what happened to me several years ago. I was flying a demonstration at an EAA event, and due to the nature of the runway (which was not trimmed for such small wheels) on my first landing, my wheels got caught in the rough turf and the plane nosed over. On the next two flights, my engine quit shortly after take off. A post-flight inspection reveled that on the nose over, the clunk had whipped itself forward toward the stopper and got stuck there.

So DON'T worry about it!