No matter what I do, I can NOT get rid of the bulge under my fondant. It's between the layers of the cakes. I have made a dam around the filling, used stiffer BC, chilled it, used thicker fondant, let the cakes have ample time to settle. Grrrrr, I don't know what else to do. It's to the point that I'm considering giving up cakes period because I can do anything else when it comes to cakes, but I cannot have a 'smooth' finish.
Try using the "cake spackle" method! You essentially crumb coat your cake with a mixture of bc and crumbs- sort of like cake ball mix, but more icing. this makes a stiffer wall and more solid surface to fondant. Another option is to start using ganache instead of buttercream under your fondant.
Are you putting your dam 1/4" from the edge. I try to stay away from the edge with the dam, then when I put the other cake on I fill in the gap.
Two tips. Make sure your cake is level - really level before you dam and fill it. then after placing the top half on top of the filling, press down on the cake to eliminate any air inside and to squish out any additional. filling that you can then scrape off before applying the fondant.
When are the bulges appearing, after the cake sets or after moving it? Could it be a support issue. It sounds like you've tried everything possible concerning the frosting. Don't give up there's too much fun to be had!
Do you let it set well before you cover it with fondant? Also, I don't remember her name but a forum superstar on here uses a tile and some heavy books to settle the cake faster.
what is your cake recipe? if the cake is too soft, it will itself compress ( not the filling) and cause that no matter what "right" process you use. HTH!