If you've got enough activity, then the reason for the log not being reused can easily be log backup all the time, because by the time one log backup has completed there's more of the log that's waiting for the next log backup.

Also the status doesn't always update immediately. Sometimes requires a checkpoint for the status to change (not for the underlying log to be reusable, just for the status in sys.databases to change)

Why are you concerned? Is the log growing? Are there large portions of the log not being reused?