I had some major issues with this before. Between my mums phone and mine, it turned out the new handset wasn't recognising her number properly after porting (it would send Chat messages on the original number of the SIM), a new SIM solved that. Between a friend and I, he accidentally turned it off somehow.
Have you definitely got Profile Sharing on in your Contacts app? The Chat Message toggle is definitely on in Messages Settings? The "Send as Chat" ption definitely there in the individual message chain settings?
Thanks for the reply. Yes Profile Sharing is on. Yes the Chat Message toggle is on. And the Send as Chat option is definitely there. Also the Rich Communications toggle is on. I've been in touch with Samsung on Facebook Messenger and they can't help either.