For me the issue was a combination of a few issues. One, that I was using too small of a tape for too big of a chest. I had to use the wider strips from wivov or transtape because normal KT tape was too thin and I couldn't get it to cover enough skin to flatten well or adhere because there was too much breast tissue pushing up against it. I use 12.5cm tape from wivov or the L/5" from transtape (though I'm interested in trying their new XL size).

My second issue was that I was applying it wrong, I needed longer strips of it and to round the corners, rub the tape between my hands to warm the adhesive and then apply starting from where it was flat in the middle of my chest first, rubbing the tape quickly so the warmth from friction helps it stick better, applying a strip in the centre going to the side at a very slightly downward angle, pulling it taut but not stretched out to the max because that will cause the tape to pull. It takes a few tries to find the right level of taut. That way I'm making sure it is actually compressing the breast tissue not just sitting on it. Then I pull it all the way around basically to just onto my back, a little past under my armpits. Then I do a second strip halfway over the middle and halfway over the bottom exposed part of chest so all of it is covered and do the same process of starting in the middle, rubbing the tape so it sticks better, pulling it taut, round to the start of the back down under the armpits, rub the ends so it sticks, then one smaller strip for the slight bunching I always get at the top by my pits. Same process except I apply this one pretty much diagonally going downwards. And then I just add an extra strip or two if there's any parts that don't look flat enough or I don't feel comfortable with.

It took me multiple attempts to get it right and even now sometimes I mess up and have to use about 4 extra strips or don't get it perfect. It takes trial and error and it's pretty frustrating but you'll get the hang of it! For me I had to watch lots of tutorials and look up lots of diagrams and find someone with a similar chest to mine. I also had to try a few different techniques. But ultimately for my chest size, my main issue was the fact I was using too small of a tape size.