I recently made a 3 tier cake and used a thick layer of buttercream under the fondant. I used plastic supports in each tier and when I stacked the cakes, the buttercream bubbled and made the fondant look uneven.
Should I have used a thicker consistency of buttercream? Or maybe a thinner layer (although I want it thick for those that peel the fondant off)? Maybe my supports were tall enough and the weigh still affected each tier?
Sounds like your supports were to short. If the supports are not the correct height the weight of the cake will rest on the tiers below and your cake layers will bulge. I use the Stress Free cake support system for stacking cakes. It's wonderful.