Jade from the Google Maps team posted in the Google Business Help thread saying that owner responses are not working again.
This would be the fifth or so time that this feature has stopped working since it launched about two years ago.
Jade from Google said:
We're seeing an issue with owner responses to reviews not posting, and we're working on it! Sit tight, and I'll post an update to this thread as soon as I get news.
Forum discussion at Google Business Help.