Hi- I made this cake for my niece baby shower- 

The bottom tier  was made of sponge cake and pineapple filling and the top cake chocolate with butter cream Since I knew it was hot and very humid I used wilton hot and humid crusting buttercream.. Both cakes were delish!  But I dont know what I did wrong. 

As you can see from the pic the bottom tier started sinking- plus it was VERY Humid in my apartment  when I put the top tier- I stacked them with starbucks straws... I know stop being cheap... but Ive used it before w/o problems.  I had bubbles  in between the fondant and buttercream - like the fondant didnt stick to the cake.... and the cake was very unstable.... it moved A LOT... so much my brother asked if I made it out of Jello...lol

 Is sponge cake stackable? 

That's such a cute cake - so frustrating!!

1) Did you let the iced cakes settle before covering with fondant?

2) Did you use a dam for the filling? I use a dam that's so thick you could make 3d shapes from it

3) Did you leave room for the fondant to stretch at the bottom? When it's especially humid, I cover the cake in fondant and try to wait a couple of hours before placing the bottom trim so I can trim it if need be.

It looks like the bulges were mostly where the pineapple filling was and the balls may have been a bit heavy. It might have helped to place support under the shoes and rattle so they wouldn't push the cake down.
 

If you baked a real sponge cake (from scratch) that's your problem. Sponge cakes are not firm enough cakes to put fondant on. Most times sponge cakes are used with whipped creams or mousse as frosting and not even butter cream which can weigh it down. Sponge cakes are used in cake rolls because of their flexibility and lack of structure.

Thanks for responding howsweet. Homesweet I did all of the above but in the future I will put support where the rattle and booties are or any heavy object thats a great idea. Thanks! 

@stitches, Yes, I made the sponge cake from scratch. After making the cake I looked into it and read some articles that say sponge are no good and some that say sponge is fine for tier cakes. 

The articles that say it's fine for tiered cakes definitely weren't thinking anyone would cover them with fondant. Fondant is too heavy for a sponge cake. There's no way a sponge cake can hold up the fondant on the edges and sides.

 It really wasn't anything you did wrong decorating, just picked the wrong kind of cake for so much decoration.

I think your correct. Lesson learned.  

Thanks for the info and outlook!