We are having the same issue. Here is Google's email response:
"Hello,
If I'm understanding the inquiry correctly, it sounds like the user is having trouble responding to reviews because of multiple accounts on the same listing. Having multiple owners on the same listing can cause a problem with being able to respond to reviews.
In this situation, I would advise you to delete the listing from one of your accounts by logging in and clicking the link "Delete" and choosing the option to remove the listing from your account. You may have to wait for that delete to register in our system and for you to become the only verified owner. This usually takes 1-2 weeks.
Please let me know if there is any additional questions that you have.
Sincerely,
Kristopher C.
The Google AdWords Team"
Our issue stems from the fact that when Google merged Plus and Places (we had separate logins for each), they merged our two logins to the same G+L account. This caused our account to be flagged for TOU violation causing us to have our reviews deleted and locked us out from being able to respond to reviews. We also had a secondary issue due to the fact that there are two separate dealerships that share the same address (same owner).
We went in and changed the address to the dealership(s) (Added and "A" and a "B" to the address) and this gave us back half of our reviews. As of this morning, I deleted the old Google Places account and I'm hoping this fixes our lack of ability to respond to negative reviews. Google stated in a previous email (see below) that this could take up to 2 weeks for their system to fix once I removed one of the accounts.
"Greetings,
Thank you for contacting Google.
The two accounts you are referring to are actually sharing the same listing. Being they are the same business our system has automatically merged the listings together. Being it is "owned" by two accounts it can cause issues with replying to reviews. In order to fix this issue you will need to log in on the email address you do not want associated with and delete the listing from the account. It may take some time for this to take effect, being the change will not go through until the next update it may take 1-2 weeks. Thank you for your patience while we looked into this.
Sincerely,
The Google AdWords Team"