Converting video to mp4 and wont play?

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Hello there, I am runing a video website. Those days we get point to make videos of site mobile available. All's done BUT ... here's one big but. When I edit the php code which use ffmpeg to convert videos to convert them in mp4, becouse this file format i wide supported by most mobile devices, I test it ... I have no problem with iPhone and iPad or any windows pc, the problem it is that: video don't start at latest android ? Why ? The converting command is this:

/usr/bin/ffmpeg -i /home/user/public_html/files/conversion_queue/137940558140573.flv -f mp4 -vcodec libx264 -r 25 -s 854x480 -aspect 1.7791666666667 -vf "movie=/home/user/public_html/logo.png [watermark]; [in][watermark] overlay=10:10 [out]" -acodec libfaac -ab 128000 -ar 22050 /home/user/public_html/files/videos/137940558140573.mp4

If someone can help me, or give me opinion, that will be great!
Thanks!