Ah - the bane of sensor adhesive. Undoubtedly, this is the life limiting component of sensor use. The technical support people and Dexcom representatives will advise adhesive augmentation with preps such as SkinTac or Mastisol. However, if the sensor won't stay on you, this is unlikely to make a huge difference. Though they are indeed waterproof, the adhesive tends to break down for some people. I would imagine it has something to do with personal physiology. With Dexcom's original system, the 3-day system, included what were termed "shower covers," which were pieces of saran-wrap like material (similar to the covering for IVs if you've ever had one) with a super strong adhesive around the perimeter. Oval in shape, these shower covers will completely block your sensor adhesive from any moisture. This makes a tremendous difference for adhesive life for ALL sensors. I place a shower cover over my sensor (now the Seven Plus system) immediately after insertion and only change the cover when it starts to come off (usually 7 or 10 days). When 14 or 17 days are up and my sensor is dead, off it goes.
It works tremendously well. I believe you still must order the shower covers through Dexcom and might have to call for them. If they no longer carry them, ask your local representative for them (even if they live a great distance away, they could mail it). Shower covers cost $5 for 10 covers.
Another advantage of these shower covers is that they smooth the contour of the sensor so its not tugged at by clothing or as obvious with fitted clothes.
And for sensors falling off, be certain to call Dexcom with each one that falls off before day 7 as they will replace it free of charge. ANY sensor that fails or falls off or is not working prior to day 7 will be replaced if its within expiration date and the troubleshooting has been performed.