A good encoding tool is handbrake.fr/.

Try also to add the correct MIME type to your  htaccess file:

AddType video/mp4. mp4

maybe this will help.

Well
my video is well encoded in h264 format. I just reencoded it with handbrake.
but still the same problem...
the mime type is in htaccess...

maybe I should reencode it in aother format ? but which one ?

Try with .mov or .avi and let me know how it went.

needed a plugin if i put in avi format.
so I choose flv (quality is less good but it's ok)
and it works :-)
thanks for your precious help Yiota

No problem.
Note that the .flv format won't work in mobile or tablet devices that don't support flash.