So, bear with me as I try to zero in on the issue you're having.
I've been living with my ostomy since 2013. I've gone thru numerous trial-and-errors in that time but, I think I have finally settled on a solution to the weak link in the ostomy chain; the flange.
Firstly, I use Cavilon spray and then a barrier ring between my skin and pouch. By limiting the amount of waste that comes in UNDER flange, it helps to prolong wear time. I currently get between 4-7 days between changes; depending on the usual variables. I hold in place it the palm of my hand over the pouch, letting the heat activate the adhesive. After a couple of minutes, I use barrier strips around the flange edges, again, using body heat to let the adhesive activate. Next, I cover the flange with a Brava 4x4 protective sheet cut in half to maximize use. These Brava sheets are not water friendly and they need to be protected. So, stay with me here, I use a Duoderm Extra Thin dressing; also cut in half. Ok, after all that you'll have a pretty good seal that will not be as prone to leakage a what I call “wardrobe malfunctions”. All of this extra “work” is to protect the pouch flange. As added step I use a freeer bag, and use bandage tape for the top and sides to minimize exposure to water while showering.