Flange Sizes

Hi! I've been pumping to try to increase my supply-- it never fully came in. The lactation nurse I've been seeing set me up with a Madela kit that I've been able to use on the one I'm renting. She has me using the 24mm flange but when I switched to the 21mm, my supply increased on one of my breast's. I finally realized I need to measure my nippled to find the best size. They both measure at 17mm. I'm a little annoyed because I feel like the nurse should have considered that, but I'm not too annoyed because I've learned so much more from her than my OB lol. Anyway, what have been your experience with flanges? Do I keep using the 21mm? Should I invest in a 17mm or 19mm flange? Keep using the 24mm on the breast that always has a bit output?? Thanks!!