I recently have had a problem with the froting wanting to slide off the side of my two-layer cakes. I am usually proud of how moist my cakes are, but can they be too moist? I use a homemade buttercream frosting using shortening. I do not let the cakes stand out to "air out" but usually keep them covered under plastic until I'm ready to frost them. The frosting is usually pretty thick to cover the cake crumbs, but don't know if the cakes need to sit out longer, don't want to dry them out; if it was the humidity that day, it was raining; or if I need to change my frosting recipe, which is the favorite of many of my customers. Any comments would be appreciated.

You are on the right track. Your cake is probably just a little too moist. Either that or your frosting is too thin, I have noticed on my own cakes that when the icing is too thin it slides right off. Also make sure that you're not applying too much icing as that can tend to make it slide as well.