Missing fonts in Illustrator on Mac

Hi,

I've downloaded some fonts and placed them in my font file (ie. on mac-- library/fonts) but sometimes they show up in my Adobe suite (I have CS6) and sometimes they do not.

I've tried restarting my mac, I've tried moving the individual ttf. file from the folder it was sitting in directly into my font file but still does not work.

Could anyone please help. I've searched all the forums but nothing has helped.

Thanks