I use the Wilton buttercream recipe when I decorate cakes.  I have started making stacked and tiered cakes. When I try to do buttercream decorations on the sides of the cakes, sometimes the icing decorations seem to "slide off" the sides of the cake.  Do I need to use a stiffer icing consistency?  Thanks for any advice.  I really appreciate it!  :-)

YES!

Use a stiffer icing on the sides as well so the "base" is firmer. Then allow to crust of refrigerate to avoid the "slide"

Thank you so much! I will try this on my next attempt!  :-)