Large Lunette and Small Yuuki both leaking

So, I started out with a large Lunette, which more often than not would leak at around 5-7,5 ml. This went on for a year, so I finally bought the small Yuuki, hoping that a smaller and squishier cup would maintain its seal, but this is the first day of my period and no such luck, it's the same as with the Lunette, leakage at the same amount of blood too.

I just can't figure out the cause of the leaks, it's usually fine for the first 1-4 hours (depending on my blood flow) and it never ever leaks below 4 ml, so the cup seems to be positioned in the right place. Maybe something's causing the cup to lose its seal, which allows some of the blood to escape the cup, whenever the blood level is high enough (i.e. at 5 ml+) ?

I'm so disappointed, any ideas what it could be or anyone with a similar experience who figured this out? I love my cups and I would hate to have to go back to tampons for day 1-2 of my period, but tampons would only leak once in a blue moon for me and had a capacity I could at least rely upon.