Hello there, jmitchell12.
The following Knowledge Base article offers up some great in-depth troubleshooting recommendations for the issue you're describing:
iOS: Troubleshooting Messages
http://support.apple.com/kb/ts2755
Particularly:
- Make sure that the contact trying to message you isn't blocked in Settings > Messages > Blocked.
- Verify that the contact you are attempting to send a message to is registered with iMessage.
- If the issue occurs with a specific contact or contacts, back up or forward important messages and delete your current messaging threads with the contact. Create a new message to the contact and try again.
- If the issue occurs with a specific contact or contacts, delete and re-create the contact in the Contacts application. Create a new message to the newly created contact and try again.
- Back up and restore your device as new.
Thanks for reaching out to Apple Support Communities.
Cheers,
Pedro.