Active track troubles

[Romenfree]

I had a play with active track earlier today and it worked fine. Returned to the field later this afternoon and I could not get it to work, the square I drew around my son to track him turned green and it prompted a "go" (from memory) but this afternoon the square I was drawing just kept disappearing. I tried several times and exited out into normal control mode several times.
The only thing that was different is the sun had dropped in the sky so lighting was different.
Any ideas?

 

[Phantomix]

that is why
it uses the sensors and camera to track, neither of which work well in low light conditions
try in the morning