Google has confirmed a bug in the Google Business Help forums, where the reviews left on your Google Maps listing in early April are now missing.
Jade Wang from Google confirmed the issue saying Google is aware of the issue and working on fixing it.
In short, any review left for your business in the time-frame of early April 2013 is not showing up.
Just letting you know that we've found a technical issue last week with reviews where some were incorrectly taken down. We're working on fixing the issue, so sit tight!
Bugs in Google Maps is extremely common, so I am sure none of you are surprised by this.
Forum discussion at Google Business Help.