No expert but you need to put loads and loads of icing on - is it a cream icing? It is best out of the fridge so it is a little harder and then just whack loads on. I have a special little icing spatula that I dip in hot water after each smoothing (but a knife would work). Sometime the icing is over a cm (or inch ) thick.
Yeah you just need to be very very generous. I have always had trouble with the corner (where the top meets the side) but I think its solved by leaving a layer to harden then going over it with another layer.
I find it easiest to put one thin layer of icing on, to rough coat the cake, sealing in most of the crumbs, and then add a second, slightly thicker layer that can be smoothed. That way, you don't get cake fragments breaking through, or icing sliding off under its own weight. You can also get plastic smoothers from cake decorating shops; I just use a big knife dipped in a tall mug of boiling water.