90% of the time, if you can't respond to your clients reviews, either because the "respond" link is missing or they just aren't displaying live after you post them, it's due to one of two reasons.

One reason is a quick fix, the other is a terrible situation that mirrors the technical issues that fell upon SAB's back in 2012.

Reason 1: Listing is claimed in multiple accounts. This is the common reason. It's usually easily correctable with a little creativity using the Troubleshooter OR using the Google phone support. Here's a recent response I received from a Google employee after reporting a missing "respond" button on a clients G+ Local page:

Thank you for getting in touch with Google!

We’ve taken a look at the problems you are having with responding to user reviews left on your local Google+ page. We’ve looked at our data and determined there are multiple verified owners for your listing.

When there are multiple verified owners, the feature that allows owners to respond to the page is disabled. However; because none of these owners were keeping the listing up to date with the latest information and updates, we have un-verified them. You are now the only verified owner and functionality to respond to user reviews should be re-stored within the next two weeks or so.

Reason #2: Technical Issue on Google's end. This is never good and there doesn't seem to be a way to fix this issue currently. Here's an example of a response (that you don't want to get):

Hello,

Thanks for sharing your concerns and we appreciate your patience. You're not being able to respond to reviews due to a technical issue and we are pursuing a resolution. At this time, we cannot say when the issue will be resolved and when you will see your reported problem fixed.

Thank you for your patience and understanding.