Leaks on heaviest day

My Mooncup UK works fine on my lighter days, no leaks at all.  On my one really heavy day, I am emptying it every 1 1/2 - 2 hours.  I am still getting leaks, even when it's only half full.   I can swear it is opening up fully and getting a good seal.  I am wondering if it's maybe because it is slicker up there on my heavy days, if it could be losing suction, or not suctioning as well.  It always moves up on me, too.  Any thoughts?