That is not to say I have never experienced a leak with the Medium LUB! I have! I wanted to share my discoveries on avoiding leaks as they may be helpful to people using the LUB, and possible even other cups. So...
Constipation. I was traveling recently and had to deal with my heavy days while in transit. Airports, planes, poor nutrition, lots of stress, limited access to toilets, and dehydration from long hours in a plane cabin. All of this will make you very constipated! Aside from being uncomfortable, I had a theory this may be affecting my LUB's ability to seal. I experienced several leaks over these days in transit.
This cycle (back home), I took care to eat plenty of fiber and drink lots of water to prevent any constipation. Without travel-related constipation, the cup sealed fine and I was leak-proof the entire cycle.
If you're trying to trouble-shoot a leaky cup, consider constipation as a factor that may be affecting the cup's seal.
Another thing I tried for this past cycle (which was entirely leak-free) was wear my cup inside out exclusively. I have a high cervix, and the length of the LUB is not a problem for me, but the texture of the inner cup is sooooo smooth. I had NOT ONE LEAK! No slobber whatsoever. It was incredible. I usually wear a pantyliner overnight or on my heavy days, but there was really no need this time.
I'm trying to figure out why inside-out may be preventing any and all leakage. If you have any theories, I'd love to hear them! Mine are:
1) Less pronounced rim. A flipped cup has a less-pronounced rim, meaning less difference between the holes and the rim. A pronounced rim might create a "tenting" effect in the vaginal walls and hold them too far from the holes to form a proper seal. This means that small amounts of menstrual fluid may be leaking from the holes. I suspect that most of my cup-related spotting has been due to leaking from the holes. For me, a less-pronounced rim helps the cup form a better seal. Fleur (which has a large rim) would start leaking through the holes at the halfway point. I have not tried to use Fleur inside out to see if this would be prevented. I suspect that folks who experience leaking from the MoonCup may be having this problem, and should switch to a cup with a less pronounced rim.
2) Okay, I really do not have a second theory.