you may be doing everything right. How many times have you made a fondant cake? maybe it was just a fluke.
the few times that I have made a fondant covered cake I only put a light crumb coat on the cake, then a thin layer of buttercream, probably about 1/4 inch thick. then put on the fondant. since everyone peels off the fondant, unless if it's homemade Marshmallow fondant (that kids and people with a serious sweet tooth will eat.)
but to counter the lack of a thick layer of buttercream on the outside of the cake (under the fondant) I split my two cakes, to make four layers of cake. then I put a thick layer of buttercream and filling or chocolate ganche on the inside of the cake. It balances it out well.
however I do live in a colder non humid climate and it may not be applicable to where you live.
keep at it, forget the failures and celebrate your successes. and take photos of your successful cakes!!