If you are sendig message with Subscriber Bulk Broadcast of Messenger BroadCaster, to enable the report of open rate,
Make sure to edit the webhook event of your app & select message_reads .
Goto you Facebook APP, From Left Menu click on Messenger -> Settings .
After that disable the BOT, & Re-start the BOT again. ( Don't Delete )
This settings is needed only one time for your app. For all future campaign, it will work.