iMessage to 1 of my contacts unable to send/receive

First, there are tons of reasons why this isn't working. First, you have to check if they have an iPhone, if their iMessage is setup and enable, if their service provider is eligible to integrate iMessages on their own devices. It might be that the apple id on their iCloud or iTunes is different from the apple id on iMessage. Please ask these questions and then get back to me. I will further assist you with the help you need.

Okay ask her to close/delete the chat you had with her previously before you upgraded to 6 plus. And then initiate a new conversation. This should work. So what i mean is what ever previous conversation you guys had before, she has to delete it and then start up a new conversation, just like sending a new message to a new contact. I hope this helps.