Hi hannahstoltz,
Thanks for visiting Apple Support Communities.
Try these steps first if you are not able to send iMessages to one contact:
- 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 from the Contacts application. Create a new message to the newly created contact and try again.
You can find these steps here:
iOS: Troubleshooting Messages
http://support.apple.com/kb/ts2755
All the best,
Jeremy