sorry but from what you told here seems indeed like you were not keeping the shutter half pressed (or the AF-on pressed) all the time.
Furthermore, if you'd recompose it would off course lose track of the subject because you're using the center point only.
I'm speaking like this because I shoot a lot of cycling stuff (with 7D) and used a lot servo. I said used because I started getting better at focusing and I don't use servo almost anymore. just single point normal focus now.
But anyway, when using servo this happened to me (that might be related to your problem):
if I use servo i can either set to use one point only or a group of points. if I use a group of points it's obvious that I MUST keep the moving subject inside the "box" of points otherwise my camera wouldn't know s*** of where the subject is. If one point that's even more difficult because you have to keep your point always on the subject moving otherwise the camera wouldn't keep tracking it.
Thus I started more and more not to use the servo. I mean I use it in very specific conditions where I NEED servo. Then on other situation I prefer one point and keep focusing on my own. That's why I found nooo issue using a 5D for sports and that's why I'm buying one in the next weeks...I tried one of my friend and saw no issue on focusing.
You just have to use it "correctly". It's in quote marks because my use of focus is not the real correct one. Meaning that on my 7D i use center point almost 98% of the time and get spot on result everytime. it's just knowing what you wanna do and how to do it.
So for instance in you example. if you use servo one point just remember to keep the spot always on the model and to not move (recompose) otherwise you (the cam) lose the subject. If you wanna be a bit more able to have better composition while using servo you have to use the box with all the AF points, but this options can bring you to lots of collateral troubles, like camera not focusing on what you really want, or just hitting once and then not hitting the focus (happens to my 7D too, so it's not a 5D problem only).
Either way I personally would use the single point focus and not servo and try to be fast focusing the subject coming at me. I do it for object coming directly in front of me, side, and also panning....for panning I only use one point af and no servo most of the time and get say 90% great-keeper result. It's just practice.
Hope it helped