Jwplayer issues displaying a video

I hope someone will be able to find a solution to my problem. It's really hard to explain all the details but I'm gonna try to be as simple as possible.

I'm using JWPlayer to display a video on 2 different websites. On one of them, everything works great, but on the other one, the video doesn't launch. After checking the code with Firebug, I saw that one parameter is not created : < param name="flashvars"...... >

The 2 websites are developped the same way and the scripts are exactly the same.

Is there a reason that would explain why this parameter is not created on the "broken" website ?

Thanks to anyone who will take time to check on my problem.

Best.