Solved! Go to Solution.
@Carolmc: To troubleshoot the issue, please open the Messages app > Tap the 3 dots in the top right > Settings > Chat Settings > Select Default Messaging Type > Text or Multimedia Messages > Resend Undelivered Chats > As Text or Multimedia Message. This is just to ensure that the affected contacts are not experiencing any compatibility issues with any chat features you have enabled. You could also try enabling Mobile Data to see if this helps, in case the messages are being converted to MMS format.
If the issue persists, can you confirm if you're still able to make and receive calls from the affected contacts? Do you get any error messages when attempting to message them?