Q: iMessage Never Received by 1 Contact

This could occur if the recipient goes in-and-out of WiFi and/or cellular coverage capable of handling iMessage.

Is this person using an iOS device? Sometimes Android or non Apple users may have forgotten to deactivate iMessage on their phone number so they keep getting iMessages despite not having a suitable device.

Get help if you can’t receive text messages from an iPhone - Apple Support

They can also deregister without an iOS device…

https://selfsolve.apple.com/deregister-imessage/

You can also look at the contact information in the Contacts app. iMessage will send to any 'iCloud account address' or phone number so you may want to move the phone number for this person to a new contact, so you can message to them directly over SMS using the name of the new contact (e.g. John Doe - SMS). 

A last resort is to remove their email addresses from the Contacts to try to get Messages to ignore it. Frankly there could be a simple 'always send messages to this person as SMS/iMessage' option in Messages or the Contacts app, so post feedback to Apple & cross your fingers.

http://feedback.apple.com/