I recently made a waterfall cake with fondant & I used piping gel for "water". Well, the piping gel was fine at first... I went do other things then when I came back to the cake I saw white spots of fondant coming through the gel. At first I thought I was just seeing things, but the longer it went the more obvious is was. The gel was sliding down the sides of the cake. It came out fine, but I had to constantly keep applying & smoothing the gel.
Anyone know why it was doing this? At first I thought it was because of gravity, but I've done this same thing before, but on buttercream icing, & it didn't slide down. Maybe the gel doesn't stick to fondant as good as it does to buttercream?
I sure hope y ou got an answer to your problem. Here is my take on it:
I'm sure this has to with it being on fondant. I can't remember ever having such a problem but I bet if you put down a layer of b'cream, then the gel the problem will be fixed :)