Sorry if I'm assuming too much; I'm not sure how old your O-ring is, and I think no-one else has mentioned it, but you should keep your O-rings very lightly greased with a silicon grease designed for this type of thing. Until now I have always used the grease that came with my 2 Canon housings. I make it a rule that every single time I open my housing (ie break the seal) then I remove the O-ring (I've used an old credit card but I love the guitar pick idea!), wipe it thoroughly with a clean lint-free cloth, also wipe the housing part where the O-ring sits - inspect both visually for any stray hairs, dirt or defects. Then I put a tiny amount of grease on a finger (about the size of a tiny grain of rice), spread it on the O-ring a bit then pass the whole O-ring through my fingers several times to coat it. Then it goes back onto the housing (in a different orientation, so kinks don't form) for a second inspection & close. I start with clean, washed hands of course! Its a ritual I go through, its how I was taught and now I'm almost superstitious about it! Whole thing takes just a couple of minutes... I also never open the housing unless I know I can clean it and re-seal it like above - if I have to open it, then I don't use it underwater again until its re-sealed.
Good luck!
-Durwin
