I am going to have a play with active track hopefully this weekend, but from the you tube tutorials I have seen, one of them showed that if there is poor contrast between the subject and rest of image it can't track, and mainly if you are simply too far away it can't get a fix either. It looks to me that you might be too far from the bikes for it to track and also the shot from over the fields seems to be a spotlight mode track? IE it tracks the subject in the centre of the picture but the drone stays put.