ATVfiles wrt subs

I converted an mkv to m4v using handbrake.
I added an srt file as sub track.
On my appletv in atvfiles I could see the subs or activate/disactivate it by long pressing the central remote button.
No problem here.

I then converted a divx to m4v using handbrake too, and even using exactly the same setting.
I also added an srt file as sub track.
Altough in QuickTime, the subs appears,
on the appletv in atvfiles I cannot see the subs and cannot activate/disactivate them !!??
Here is the problem.

Can someone understand the issue?
Can someone tell me how to overcome it?