This seems like a frustration for many who do SEO for local business. The Google Plus aggregation of reviews is vaguely created and out of our control. Or is it?
The most I can find about it is at: https://support.google.com/business/answer/3474120?hl=en&ref_topic=6109351
I have clients in the accommodation industry who are reviewed thoroughly on similar sites to one another. Some of these clients have 100+ "Reviews From Around the Web" where as others are stuck at one or two reviews. My thoughts are that the business name, address, and phone details would allow Google to link the review with the local listing. I'm not sure and doubt this is just the case because the low-review listings have consistent NAP details.
Have you been able to get reviews aggregating for a local business in their Google Plus?