Menstrual cup leaking?

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So, I have been using the mooncup (US) for several cycles now and although I absolutely love it when it works, I find that I frequently have a day or two that it refuses to make a good seal. When I first insert it, I ensure it is fully popped open, I can spin it easily inside my vagina, and it is as low as is comfortable. Everything is hunky-dory when I leave the restroom. However, I find that throughout the time I wear it, it slides much farther up. I never have a problem removing it, but believe that is why it stops sealing well and causes leakage. 
Do you have any tips for helping it stay in place? Or do I just need a longer style of cup for those days?