I selected to send SMS instead of iMessage to a contact once, and it won't switch back, Why?
The easy way to fix this is to look at the contact card. (Assuming the issue is on your side of the sending)

You could temporarily move the phone numbers to the notes section and delete them. Then sending a message has to go via iMessage if their email is registered for iMessage.

If not, just add back their mobile number and initiate an iMessage to them from the address book/contact card view. Tap the chat bubble next to their email or phone to initiate an iMessage transmission.

It's often required to start a new thread if you keep sending to the wrong email/mobile number since Messages wants to hold on to the way the last message was sent and not jump back to iMessage if the other person's AppleID hasn't claimed the mobile number, so keep in mind the problem might be on the other person's account - not your address book or contacts.