Stupid Question/Bra Rant

Okay so I realize this is probably an extremely stupid question but I’m gonna ask anyways lol..

So I have been fitted multiple times for a bra at different stores, always buying the correct size, and good quality bras. Well I’ve tried D cups & my boobs always come out the bottoms of my bra, i sized up to a DD & the same thing happened. I’ve tried different band sizes & it’s always the same thing. I’m ALWAYS adjusting my bras. Always lifting my boobs back into my cups only to have the same problem 10-20Minutes later. I am sooooo sick of it! Anyone else have this problem? Any advice? Insights?