Hello bymeown. Thanks for the post as you have grasped the problem so consicely and I am looking forward to hearing lots of replies.
As I have a colostomy, I could not put the problem down to urine but it might well be sweat, as I do a lot of that.
I have a peculiarity that I'll share, just in case someone has some ideas. one side of the wafer usually stays stuck fine. The other side does not and that is the side where it will leak and come away from the skin. What I have noticed is that when, after removing the wafer, I come to remove the adhesive, there is heaps of it stuck to the skin on the side where it doesn't stick so well and hardly any on the side where it stuck perfectly. To my way of thinking, this indicates that the adhesive is sticking to the skin but actually coming off the wafer. Interesting! but I have no idea why.
Best wishes
Bill