Do you only put a crumb coat of icing under the fondant or a little more?
I usually do 2 coats. My crumb coat and then another one. You dont want to see the crumbs thru your fondant.
Your fondant will show every little detail if there is a bump, crumb, etc. under it...so you may want to do more than a crumb coat...Also,...Look in the forums for those who are using chocolate clay under the fondant...it makes the fondant so smooth...you still put a small amount of buttercream under the chocolate clay...you will not be disappointed.
I use ganache under fondant which sets up nice and firm, so I can put as much as I want and it's still coverable. That said, I tend to make it only as thick as I have to on the sides, and use it thicker as a filling and on top...my reason being that if it's a large cake then that way there won't be a huge difference in the amount of ganache you get on an inside slice v's an edge slice (if that makes sense -?)
Thanks for the advice!