I had a little trouble too. In fact I just found best fit with the rubber tips by going smaller. I kept going larger with foam. And with rubber even the largest wasn't sealing - till I figured out I needed to go smaller.
I don't like having to squeeze the foam then hold in place while it expands - I liked the black foamies on my Shures in that regard. But just fiddling around and found best fit so far by going several sizes smaller on the rubber tips.
Now sound is nice and warm. better bass. I guess I wasn't getting deep enough fit with too large tips for my ears. I started with 12.5 and went up from there at first. Then I tried the rubber - smaller than 12.5 - and wow. This tips fits just right - reminds me of the Three Bears ; )
Give it a try. Hope it works for you.