It will probably come down to two things. Either the animation is looped, or needs to be looped, or you are constantly setting the animation.
checks Key is down - set animation walk
checks Key is down - set animation walk
checks Key is down - set animation walk
checks Key is down - set animation walk
checks Key is down - set animation walk
checks Key is down - set animation walk
Each time the run time hits the event it sets the animation again so you would only see the first frame or so.
So there are a few ideas, see how you go, if you have any trouble post your cap and I will take a look.