Fondant sliding off cake

I’ve done quite a few fondant cakes but had my first real disaster yesterday and I’d love to know if this is something the experts here can help me with. It has been unusually warm and humid for this time of year so I think that may have contributed but I was working on it during the evening and didn’t think it would be an issue.

I ganached the cake the day before using the upside down method and it looked perfect – straight sides and lovely flat top, in fact I thought it was the best I had ever done. I left it to sit overnight and it was firm to touch when I began preparation for the fondant. I gave it a very light brush with water – as I have done many times in the past and got my fondant ready. It was very fresh out of the tub and rolled easily.

The disaster happened when I put it on the cake. It was like watching the fondant slide down the sides of the cake. It didn’t rip or crack, just ended up about a cm thicker at the bottom and no matter what I did it kept ‘welling’ at the bottom. I decided to leave it alone after working on it for a while and put another thin coating on top today to try and even it out. When I cut into it, the ganache was thicker at the bottom of the cake. I have been using Cadbury Dairy Milk for my ganache but I think they’ve changed the quantity of cocoa in it so it’s too low and doesn’t set up, seemed quite firm but then went to mush under the fondant. I have used it in the past successfully. What do others use for ganache, I’m in Australia?

I’m so disappointed as I’m trying to build a portfolio of cakes so I can eventually start my own business, I’d love to know if others have experienced this and what you did to remedy it as it’s never happened to me before and I’m feeling quite dejected about it. At least I was able to retrieve it a bit so I could complete the design and get a few photos, but I’m so glad it wasn’t for an order!