Are the bags they are currently trying all from the same manufacturer. It could be that she is allergic to that type of adhesive and may be good idea to try a different manufacturer and see of that makes a difference.
For the moment though the skin needs to be protected to allow it to heal. There are barrier wipes and creams and think available in a spray that will not sting but will provide a very thin barrier over the skin to allow it to heal.
You need to make sure the nurses are proper trained stoma care nurses so they have access to the latest appliances.
Hopefully you'll get on top on this soon and your mum will be feeling better.
Crohn's dx 1989, loop ileo 1992, end ileo 1992. Arthritis developed 1990 Stoma abcess 1995. Azathioprine started 1995. Panprocolorectomy 1999. Stillbirth 2000, antiphospholipid syndrome dx 2000. Flare up 2004, stoma abcess and strictoplasty 2004. Low blood pressure dx with impaired kidney function 2006, fludrocortisone pres. flare up 2010, steroids, Humira, codeine.