Hey @chris e​! Daniel from Customer Support Engineering here 🔧 

This happens when a user is unable to write into a conversation. This is typically set up in rules to control outbound conversation volume in the Messenger settings. This could be because the user has to match certain criteria to write in, or because there is a control on the number of conversations a user can open in a period of time. You'll want to check your Messenger settings to see what the cause of this could be.

Hope this helps!