Hi Leslie, just know you are not alone in this struggle. Lots of cake artists deal with this, especially when first starting out. There could be a few different reasons for this. A common issue is buttercream consistency. If it’s too unstable/loose, and American buttercreams tend to be, it will start to squish out as the cakes settle. Cooked buttercreams, like Italian meringue and Swiss, are far more stable and taste incredible! Another issue, as I said, cakes settle, so if you can torte and fill your layers then wrap in cling film, place a book or something on the top to help accelerate the settling before you ice the outside. Structurally, ganache holds things in better than buttercream, so if you fill your cakes with buttercream and then use ganache on the outside, you’ll find that it does a better job of reducing bulges. It also helps to have perfectly torted/level layers before icing with buttercream. If you’re interested I have a few videos on my YouTube channel that explain some of the basics of torting, leveling and filling and also how I cover cakes to get sharp edges. My channel is Cake with Pamela Jane. I hope that helps. You got this!
Thank you so much for you advice, it has been really helpful, I will be trying all your advice on my next cake
Again thank you Lesley