Okay-so I am not sure what I am doing wrong. Doesn't matter what color fondant I am using, but after I lay my fondant on the cake you can see the seam under the fondant. So I use buttercream as a filling in between my two 8x2 cakes and do a light buttercream of the entire cake. Let crust a little and viva paper towel to make smooth. I put on fondant under cake eventually you can see under the fondant of where the two cakes come together. I'm not sure what is going on. It is bubbles out at the seam. Is my fondant not thick enough? Is my cake shifting as it settles? I will try to upload a few pics. Some people don't notice but I do.
couldn't get pic to upload
It sounds like you're not letting your cake settle.
Read this thread. http://cakecentral.com/cake-decorating-ftopict-633571-newest.html+trick
Let the cake settle at least 24 hours, then trim any bulges all around the cake. I put a generous layer of bc or you can try ganache also under the fondant.