Can't make it work.

The first time I got a cup I used it for a backpacking trip that I conveniently had my period on. It took a few tries to make it work but then it worked great! No leaks or anything. Well every period since then I try it and sometimes it works, sometimes it doesn't. I've watched videos about how to make sure it seals but sometimes it still leaks. Are there better brands I should look into? (Using Diva Cup from Target). I'm not sure if I need a different size because sometimes the one I have works without issue. I have an extremely heavy flow the first two days and flood right through tampons so I'd love to figure it out!