Favstar’s been getting a lot of criticism recently for not sending notification tweets from @FavstarOfTheDay and other accounts. This has been odd, as when I’ve checked, the notification tweets (@mentions) have been there.
It turns out @Twitter for iPhone will sometimes not show a mention on their Connect -> Mentions tab. This seems to happen when an older mention has been retweeted by another user, which takes the place of the mention that should be there.
These screenshots show @Favstar’s mentions. First, a screenshot from Tweetbot for Mac. Note the mentions from @BauDaLayla and @PaulShipper.

Now the screenshots from Twitter for iPhone. Note that those two mentions don’t appear where they should here, but have been replaced.


And finally I checked in Tweetbot for iPhone, which does show the mentions correctly:

Twitter’s website at Twitter.com also showed these mentions correctly, when clicking on connections, then mentions.
If you rely on receiving your mentions reliably, I would recommend using a second app to check your mentions until Twitter for iPhone is fixed.