My iMessage isn't working

Hi eadaoin_m,

Thanks for using Apple Support Communities. Based on what you stated, it sounds like messages works but not with one group contact. I would recommend that you read this article, it may be able to help the issue.

If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

Delete existing conversations

If you can't send messages to a certain recipient, delete any conversations you have with them and try sending a new message. When you delete a conversation, you won’t be able to recover it. Make sure that you take a screenshot or forward any messages that you want to keep.

Restart your device and check for updates

1. Restart your device.
2. Check for a carrier settings update.
3. Check for an iOS software update.

Cheers,

Mario