Rammus Taunt Is Not OK

I don't often play rammus, but when I do, I occasionally see a taunt not work immediately. By this I mean you taunt the enemy while they run away, and if you're able to taunt them they keep running for some random duration. Sometimes they keep running or the taunt brings them back just a bit. It's shown mostly among melee champs, but a few ranged have also cheated me. Either way it's problem and screwed me over in a few ganks. I'm not tech-savvy enough to make a gif of the event, but surely other people deal with this.