AYou need to let your cakes settle for about a day or put something heavy on them for a few hours. If the cakes haven't settled the weight if the fondant will cause the problem you're having. Another cause is your dam can be squishing out, press on the cake firmly after putting on your second cake layer in order to pre-squish. It also helps to put the cake in the fridge for about 30 minutes to help the icing firm up.
AThank you that is very helpful & I will definetly try it !!!:)