Icing sponge cakes with Sugarpaste

Hi,

I have been icing cakes for a little while now, but I am still having trouble when icing a sponge cake with Sugarpaste. I generally use chocolate or madeira cake, and sandwich them together with buttercream icing. But when I cover them with the sugarpaste I get a ridge around the middle of the cake where I have sandwiched the two cakes together. I don’t want to stop having icing in the middle and don’t want to make the icing too thick. Do you have any suggestions?