This morning Gmail did an update that stopped ActiveInbox from labeling conversations (you will have perhaps seen the Labels dropdown appear after clicking an AIB button, but no label).
This update fixes it. It should update automatically (definitely in Chrome), but you can expedite the process by installing straight from www.activeinboxhq.com/install.html
As ever, sorry for the inconvenience – Gmail wobbles are what we are perennially on guard for!
(ActiveInbox 4 power users… an update is coming next!)