Thumbnail not display

[Fancham]

Hi,

I have installed Piwigo Version 14.4.0 and VideoJS 2.9b and I can watch my videos on Piwigo but no thumbnails are displayed in the gallery. I have checked in videos folder, I found pwg_representative with jpg with same name as mp4. All seems OK but no thumbnails are generated when I synchronize. I have this message :
Synchronization results
6 video(s) selected
6 video(s) with metadata added
6 poster(s) created
0 thumbnail(s) created
0 warnings during synchronization
0 erreurs survenues durant la synchronisation

Thanks for your help.

Regards