I used to have this problem quite frequently, but no longer 
" alt="" title=""> I will still occasionally pop into the bathroom and adjust, but for a year now I just haven't been wearing bras.
1. Are you sure that you're wearing the correct band size? I used to wear 32 band size bras, before I realized that 28 band sizes existed and were much better for my small frame. A band size that's too large cannot properly hold my breasts and flattens them out, pushing the tissue back into my chest and into my armpits instead of giving me proper support.
2. Are your cups large enough? You may find yourself popping or spilling out. For 1 and 2, use this calculator: https://abrathatfits.org/calculator.php
3. Do you have migrated breast tissue? I regained 2 entire cupsizes in 2 months by switching to better bras in my correct size. Maybe there is simply not enough room and the band size is not the best in order for the tissue to stay?
4. A different type of bra might be more suited to your shape. I am not very knowledgeable about this right now, but a lot of folks seem to have fit issues with the foam cup molded bras. I use https://www.reddit.com/r/ABraThatFits/wiki/shape_guide as a reference.
For 3, I will quote this article for you below: https://bustyresources.fandom.com/wiki/M...ast_tissue
Migrated breast tissue
Migrated breast tissue, or MBT, is breast tissue that
has, over time, shifted away from the breast mound to settle in the
armpits or back, often even resulting in rolls. This is often the result
wearing a poorly-fitting bra with too-small cups and a too-large band.
However, migrated breast tissue can be corrected over time by wearing a
well-fitting bra and gathering all the migrated tissue into the cups
each time a bra is worn, using the scoop and swoop.[2][3]
There are also reports that breast tissue can migrate to the
center of the chest, resulting in poor division between the breasts.[1]
Ill-fitting bras may cause breast tissue to migrate to one's armpit, back, and/or side
Cause
One common cause of breast tissue migration is constant use of an
ill-fitting bra; specifically, for migration to the back and armpits,
bras with too-small cups and a too-large band. When wearing such a bra,
the tissue in the breast mound squeezes out the back and sides of the
cups, into the armpits. Over time, this tissue moves more permanently
into the areas into which it has been pressed.
Bras with a too-large cup may cause tissue migration to the sternum.[4]
Correction
- Main article: Scoop and swoop
Migrated breast tissue can be gradually moved back towards the breast mound by wearing a well-fitting bra and regularly "scooping" the migrated tissue into the cups.[5] Correction of serious breast tissue migration may result in a change of size,[4][6][7] namely down in band size and up in cup size.