Hi ayva288,
Thanks for contacting Apple Support Communities!
We understand that you're unable to send messages to some contacts and it indicates that you've left the conversation although it's only a single contact.
If you haven't done so already, you may consider restarting your iPhone: Restart your iPhone - Apple Support
You can also check out this resource which explains how to troubleshoot your Messages app: If you can't send or receive messages on your iPhone or iPad - Apple Support
"If you still need help
- Try to restart your iPhone or iPad.
- Check your network connection. To send a message as iMessage or MMS, you need a cellular data or Wi-Fi connection. To send an SMS message, you need cellular network connection. If you turn on Wi-Fi Calling, you can send SMS messages over Wi-Fi.
- Check with your carrier to see if the type of message you’re trying to send, like MMS or SMS, is supported.
- If you’re trying to send group MMS messages on an iPhone, go to Settings > Messages and turn on MMS Messaging. If you don't see an option to turn on MMS Messaging or Group Messaging on your iPhone, then your carrier might not support this feature.
- Make sure that your device has enough space to receive images and videos.
- Make sure that you entered the correct phone number or email address for the contact."
If you're still experiencing the same behavior, you may also consider reporting this to Apple directly here: Contact Apple Support
We hope this helps.
Cheers!
