Twitter has taken off lately with native iOS integration in Apple's latest firmware. The Twitter app is also available for free on the App Store, providing an easy way to access the service on the go. Lately some users of the app have experienced an issue where the Connect tab does not show @mentions.
Mashable explains that the mystery bug is affecting Android users, too. Although Twitter has not released a fix yet, there is a simple workaround that should solve the problem on your iPhone. Reinstalling the app or rebooting your iPhone is not necessary.