I'm managing the community with quite noob admins, and I see they don't always realize there are messages waiting for approval before replying to threads, specially when they navigate the board from their mobile phone.
And honestly at the beginning of my XenForo experience, I used not to notice...
Show sticky create button on several places with some extra features.
enable sticky Post thread button (enable it separately for mobile) on forum list
enable sticky Post thread button (enable it separately for mobile) on forum view
enable/disable default Post thread...