Any ideas why the font in the header of tweets isn’t appearing? All relevant fonts on my macbook are activated.
That sounds weird - you mean the the twitter handle (usually a bold font next to the avatar icon, above the body of the tweet)? What version of the Twitter app are you using, and on which OS X version?
I had the same problem. If you are using Linotype FontExplorer – go to Tools/Clean System Font Caches. This fixed the problem for me.
Actually the problem still persists. Clearing the cache in FontExplorer did fix problems I was having with Helvetica Neue in Messages and Safari.