Have you ever gotten crumbs in your icing?
Try chilling your cake for about 30 min prior to icing it. Do Not try to ice a frozen cake. Just chill it so it is cold all the way through. When applying icing to the chilled cake try not to over work it. Put the icing on and move on. Once all the cake is covered in icing you may move in one direction and smooth the cake.
Hope this helps….If you have questions or comments please let me know.