Wondering how long you are allowing it to settle before you put the fondant on. I level and crumb ice my cakes the day before, and then do the final coat of icing and cover with fondant the next day. I know some rush this process by putting a tile on top of the cake for faster settling. Also some cakes are more sturdy for applying fondant, maybe your cake was too light, its hard to know, but a few ideas. I never have that problem, I do store the completed cake in the refrigerator, so it stays firm.