Using RCS message to android still getting sent by SMS message

I’m sending message to a person that has an android and usually I can see delivered or read. No this right moment it doesn’t even say sent as SMS. I have, in settings, I choose RSC messaging. message to other friend and or iMessage work does it mean that’s the issue is connection on the other end? And can that be other reasons? I just wanna find a sustainable way to message that person.