[Wannabe]

Under of my boobs does not stretch as much as the upside.

Your question about more of the top of the breast being exposed to suction than the bottom is an interesting one. I've considered this myself. I think the solution may be to keep playing around with the cup positioning until it the nipple is closer to the center of the cup.

It often appears that the breast has more tissue above the nipple than below, but I think that's not usually the case. Of course, everone's body is a bit different. Larger breasted women may even have more below the nipple, due to the weight of the breast tissue.

I'm thinking you've not found the sweet spot for your cups yet.