A Google Business Help thread has complaints from some business owners who are trying to leave responses to some negative reviews on their business profile.
The issue started up around July 9th and Google confirmed the issue yesterday. Jade from Google said:
Review responses not appearing is an issue we are continuing to investigate.
There is no estimate time for a fix but this should come to no surprise to anyone, the fact that there are issues with Google Places for businesses.
Forum discussion at Google Business Help.