I shoot dogs quite a lot and it's amazing how it can be really easy to nail the focus with some dogs, and really really difficult with others. Markings & size play a big part; for example collies are typically very easy (they're larger, and their faces are often perfect high contrast targets for the AF) while smaller dogs with softer/sparser markings, like yours, can be tougher. Light quality is another huge factor - an overcast day doesn't help the AF one bit, and a backdrop that competes for the AF's attention isn't going to help either.
With a small agile dog like yours I'd use single point only, as noahcomet suggested, and just work really hard at keeping that point right on the dog. In this case, the dog's head has the most contrast so that would be my target of choice.
Also take *great* care to make sure the AF point is bang on target as you engage tracking. It can be very easy to get a bad initial lock if the target is quite small in the viewfinder, so sometimes it's better to wait until the target is a little closer (or zoom in closer) before starting the track. Also be prepared to disengage/re-engage tracking any time your dog changes direction or presents a different aspect to you and the AF (e.g. as the dog turns). Rear button AF is a big help here, at least for me.
Consider adjusting tracking sensitivity too. I've tried the five different positions but I keep coming back to one notch up from slowest, even when there are no intervening objects to worry about.
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