If this didn't work, try this: go to your google contacts page from your desktop computer using your browser of choice. This page might be Google's old contact management tool - if it is, on the left, click "Try contacts preview".
Once you're in contacts preview, Under "More", go to settings and check "Include everyone from your circle in the all contacts. Exclude circles you just follow."
If this is no good either (we're so sorry! 😔), add the contact you want to use Hangouts with via drupe to custom G+ circle using your phone.