After finally finding a flange that worked (convex) - I was still having leakage problems. My nurse started using paste (I call it caulk) - she placed it around the stoma, flattening it out before she applied the flange. That leaked as well so my retired Engineer husband came up with an idea that has worked and no leaks in over one month!
We put a line of caulk around the stoma; then place an identical line of caulk on the flange - we do not flatten it out - it looks like toothpaste. After letting it set up for one minute, we place the flange on the stoma, pressing down the two donuts of caulk....hold it down for a minute or so and VOILA. I have not had any leaks.
If necessary we will smooth out the caulk if it squishes out towards the stoma....sort of making a "ramp".
I hope this helps for some of you out there. Emotionally this helped me tremendously - I am no longer fearful of leaks - I am mindful that they can occur but the fear is down considerably. Any my skin has cleared up....the powder and second skin it incredible.