Girls with HRM!

I wear my chest strap under my bra band to help keep it in place (if I move it the the side it really hurts the scars I have from a breast reduction and if it's outside of my bra it falls even if it's tight enough to leave indentations) with the sensor in between my boobs, but I'm running into some problems. When I do VERY vigorous cardio sessions, it tends to slip out a bit (where the actual sensor is). And when I lay down on my back to do certain strength exercises, it doesn't pick up my heart rate as easily. I've heard from other women with larger busts (I'm a 34DD) that these are common problems. Any solutions? I feel like I should be using double sided tape during cardio, but does anyone have a good waterproof brands that they like? I sweat A LOT! Any tips for these problems would be great.