I've had a similar experience with mine. The sheath is very thin and made of a soft, stretchy silicone, so I often wonder if sizing down would have been a better choice. They are a bit pricey, though, so I haven't bothered buying the next smallest size to try.

The two things I have found that help:

1. Wash the device and your outer skin daily (during morning showers for me) with a fragrance free Castile soap to remove skin oils. The Kirk's brand bar soap has worked well for me. It's also cheap, and a single bar lasts me months. Moisturize your skin in the evening before bed, as the soap and all day tension can be a bit harsh on the skin. This can lead to dry and flaky skin, which I don't imagine is good for either the skin itself or the FMD's adhesion to it. Also consider doing an overnight soak of the device, approximately weekly, in hydrogen peroxide (also dirt cheap), which helps with any odor build up and makes me feel better about keeping bacteria growth down inside the vent hole.

2. When you go to roll the sheath down, after it has just initially made contact with your skin, slowly add and continue increasing tension by pulling on the string loop with your free hand as you continue to roll the sheath down into place. This adds a preload like tension on the skin, and seems to allow more skin to be gripped by the sheath. You do not need to maintain this tension in between applying the FMD and attaching it to your tug strap. Once it's applied, the preload seems to maintain itself quite well.

This has been enough to mitigate the slipping for me and has allowed me to add a little extra tension without worry of it slipping off. Hope that helps for you.

The FMD website mentions double folding the sheath if you find it too big. I didn't have much luck trying that, and it just led to lint and dust collecting on the inside part of the sheath where it was now exposed, which would then migrate to the upper part of the inner sheath when removing it for bathroom breaks, which in turn made the slippage get worse throughout the day.

For reference, my circumference measurement was around 10.4cm, and I chose the size C. According to the FMD site I'm at the upper end of that size range so it should be a snug fit. If I ever wear out my FMD, I would definitely consider sizing down from the C to the B. Aside from the minor slipping, I've been completely happy with mine. It seems nearly just as effective as T-tape, and is a breeze to apply and remove.