How to revert to iMessage after forcing send as SMS

Hey all, I was texting a friend and I had poor signal so I forced the message as an SMS. Now all messages are sending as SMS and it's not reverting to iMessage regardless of our signal strength. All my other iMessage conversations are unaffected and I have turned on and off iMessage and reset the network settings.

Is there a way to force send a text as iMessage to kick it back into what it ought to be doing? Thanks!