well it isn't right i don't think, just fired up another mathmos lamps and it has the same issue, flows fine though. i've also seen this is some of my american lamps
Do you mean that it is not as glossy? has lost its "shine"? or by powdery to you mean that there are particles of unmelted wax in the goo? either way it doesn't seem to have effected the flow.
indeed - i seem to have fixed the old bottle i got with my boot sale aquisition ( has a strange metal mesh at the bottom rather than a spring ), i now notice the wax has a texture visible in it rather than a shiny smooth surface - doesn't stop it working though,