Facebook has put a new law in place in order to cut down spammers and anyone abusing the messenger platform. So they have put a short process in place to keep their platform clean and spam-free. 

There is a rule when sending a broadcast message, 24+1 on facebook. As long as your subscribers interact with you, then you can message them as much as you want. 

However, promotional broadcast messages can contain ads and promotional materials, but it can only be sent to subscribers who were active in the past 24 hours and interacted with your chatbot.

After the end of 24 hours window, you have an ability to send a "1 follow-up message" to these recipients from the chatbot. After that, you won't be able to send them ads or promotional messages until they interact with you again.

Please read this doc which explains everything step-by-step: https://docs.botstar.com/en/broadcast-rules.html