I don't have enough time to search for the answer so sorry in advance! I've researched and tried different things. Thicker fondant/thinner fondant, different brands. I've read that you need to freeze the cakes and then let them defrost which I did...or frost and let them sit for 6 hours so the gasses and things settle. I level them. I have put the cakes In the freezer with the filling inside (read that will make fondant smooth because the frosting and filling will be stable and smooth around the sides) and still I'm here and my fondant is not smooth!!!! ANY other advice would be great! It's always the layers that show horribly or bulge. And no matter how much I smooth the sides and for the few hours after I cover it still doesn't smooth out!

I don't allow my cakes to settle; I've never had any problem with that.  I use imbc, which gets very hard in the fridge.  That gives me a good base for the fondant.  It sounds like you have a problem with bulging? So maybe you're using too much filling, or not the right kind.  And I'm assuming you use a dam if you're filling with something other than bc?

if your problem is something other than bulging on the sides, let me know.  Just make sure you're not using too much filling.  Refrigerate after crumb coat until stiff.  Then refriderate after frosting until stiff.  Then while covering with fondant, don't allow to sit out too long.  I put it right back in the fridge after covering and trimming.  I've never had a bulging problem.  

@Brookebakescake  thanks for responding. So how long do you put the cake in the fridge after fondant? Does the sweating mess anything up? Mostly seems like bulging....I think my buttercream isn't the right thing to use because even when it isn't bulging it's indenting where the layers meet. I refrigerate after crumb coat and then after second coat...then I leave it out after fondant. I am noticing on this cake I can't seem to get any part of it smooth..on the top there are some indents and things. It's 3 tiered and the bottom 2 are giving me pain. The top one I used a box cake with regular white canned frosting and it seems to be the better tier. The customer was ok with that for the kids at the party. The bottom 2 have mousse filling and buttercream around it and they are scratch made. Could it be the filling? And but sucky buttercream? I get the buttercream really smooth before I fondant

for smooth fondant the cake needs to be level and the buttercream needs to be very smooth.

you can't trap any air under the fondant; sometimes it looks smooth, then an hour later the trapped air will distort the surface. Start smoothing from the center top, work out to the edges.  I find gently lifting the fondant as I go around the cake, letting the weight of the fondant drape itself, then gently smooth in small circles makes a pretty good seal between the fondant and cake.  I think if there's too much fondant, your more likely to trap air.  For me, I roll the fondant out just shy of the cake diameter and use the weight of the fondant to completely cover the cake.

are you using a quality fondant? Not all fondant is created equal. Lesser quality fondant can stretch and sag after its applied.

When I make a cake, I torte the layers, and pipe a a thick line of buttercream around the outer edge of the cake.  I fill the area inside of the circle with my cake filling to just below top of the piped buttercream. This step is especially important with a softer filling like a mousse; the softer the filling, the bigger the dam I put around the edge to hold the filling in. I put on the next layer of cake and repeat filling and stacking until done.  Then I put a something on top of the filled cake that is at least as big as the cake, and if needed add weight to it. Check for level with the weight on. The idea is that the fondant is a little bit heavy and will compress the cake a bit; this step replicates that compression, and gets the air trapped inside out. This should help prevent bulges. I'll put the cake in the fridge with the weight on and let it firm up.  I'll quickly crumb coat, put back in the fridge, and then do my final frosting. You need to get this final coat as smooth as possible. It should look nice and clean, with sharp edges. If you need help with frosting smoothly, google the upside down method of frosting; there are videos on it and craftsy classes too. Once frosted, it's back into the cold until firm, and then I cover with fondant. Try to gently push the air out from between the fondant and buttercream from the top center of the cake to the outer bottom edge.  I don't have an issue with a using larger amount of fondant; I find it easier to avoid wrinkles with bigger fondant piece rolled out, but you have to smooth carefully from top to bottom. And if you find an air bubble, use a fine needle to poke a tiny hole in the fondant to push out the air. Use fondant smoothers, your hands, and if you want, pieces of acetate or thick but bendable plastic sheets to smooth the cake. I always put the cake in the fridge after fondant. I only have trouble on days when the humidity is extremely high (more than 85 or 90% relative humidity). The cake will dry eventually but don't touch it while it is wet or you'll leave marks. Some other thoughts. If you push down too hard on the top of the cake while smoothing the sides, you can cause the cake to bulge. Or if you push too hard on the side in one spot, you can cause bulging as well.  You want to smooth around, not inward. Even with all that, you might get small bulges or indents. It is just a matter of practice and experience. You'll get better as you keep doing it. Hope this helps.