UGH! I'm getting so frustrated! I did two cakes this week. One turned out perfect, no bubbles, nothing. The other, well I put the fondant on yesterday and opened the box today to start putting decorations on and it's got like this buldge. First I thought it was air, but when I "pop" it I found it was icing coming through. It wasn't where the filling was either.
So I'm trying to figure out if I'm using too much icing and/or if my problem is that I use half shortening / half butter for the icing. I'm wondering if I should use straight up shortening icing for directly under the fondant. Even though it's not hot in here at all... I just can't figure it out.
I hope someone can answer this! I had the exact same thing happen to me last week. Covered the cake with fondant before I went to bed, got up to put decorations on and there was a HUGE bubble in the fondant, not near the filling either. I wanted to cry because I thought I was ahead of schedule!!!! 
Fondant needs to be put onto a room temperature cake, any chill and you can get air bubbles. I always freeze my cakes so when I take them out of the freezer I fill and crumbcoat and then let the cake sit and settle for two hours, at least, longer if I can. This brings the cake up to room temp as well as lets any filling that is going to sploosh out do it's thing so I can smooth it down before putting on the fondant. The only time I've ended up with bubbles is when I have rushed this process. HTH! Cat
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