iMessage not working with only one contact

Hi All - I have a weird one. iMessage has stopped working for only one of my contacts, everything else doesn't seem to have this issue. When I try to send a message on my iPhone or Macbook it times out and doesn't send. This particular contact and I do a lot of voice notes and those look like they're at a mini rave when I open our text thread. I was able to send and recieve voice notes from them on my apple watch initially but their last voice note is not loading so I can play it. The whole app just really freezes up when I open the contact; I can barely type a message and even if that works I can't send the message I type.

Things I've tried -

• Restarting my iPhone
• Turning iMessage off and on on my iPhone
• Turning my Wifi off and on on my iPhone

Let me know what else I should try!