For some reason YouTube videos in my website do not get the fullscreen button, even if I added the parameter allowFullScreen set to true both in the object and embed tag.

Here's an example page: http://www.indievault.it/2011/11/09/indie-vault-alla-games-week-2011-online-la-video-gallery/

Just take a quick look at the source. The allowFullScreen param is there, but the button won't show. Here's an excerpt from the code in that page:

<object width="540" height="325"><param name="movie" value="http://www.youtube.com/v/dvYQhJwwkgA"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/dvYQhJwwkgA" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="540" height="325"></embed </object>