Have you taken it apart and cleaned both parts in the sink with dishwashing soap? No scotchbrite, just fingers. It should remove whatever it is that is causing the silicone seal to slide. A low temperature bake out in the oven will drive off moisture (100F). Then, maybe a little bit of Armorall or something like that on the helmet gasket to get that slippery?
R1200RT (R1100RT gone)
Don't be the guy who needs to pee on the fence to figure out that it's electrified.