This is an old question, but the answer is still actual so I'm posting this for the benefit of future readers.
It appears that Google Reviews can be deleted for many reasons (source):
- Your G+ account is empty or shows little activity before/after the review
- You left a negative reviews and the business owner asked for its deletion
- Your review contains URLs
- The same review appears elsewhere online
- You are the business' owner, an employee, or an ex-employee (article)
- The review was posted from the same IP address of the business
- The review was posted from the same IP address of other reviews for the same business
- The review was posted from the IP address of a SEO company
- The review was posted from a totem (tablet, computer etc.) for customers at the business' place
- You tried repeatedly to review the same place
- The business has an abnormal number of reviews, or a high number of reviews in a short time frame
- Pretty much whatever Google feels like at the moment
Some of my reviews have been deleted, too. It's very annoying.
Note the point #2: A business owner can unilaterally decide to delete a bad review, and even a bad star rating with no review. I've seen businesses systematically removing all reviews with less than four stars. When you give a star rating which gets deleted, the stars rating still appears when you look at the place from your Google account but it's not shown publicly anymore nor taken into account for the average rating. (See also this article.)
Hence I'd recommend not to bother writing negative Google reviews anymore, because they will disappear. You can still assign star ratings as a personal reminder of how crappy or awesome the place was, just for you.
As a consequence, Google Maps ratings are unreliable and misleading, so you should not consider them as guide neither wasting time writing reviews on Google. Rather, use sites that actually provide a service to customers and value their reviews - such as Yelp (businesses), TripAdvisor (hotels, bars, restaurants), or GlassDoor (companies). All these sites don't allow reviewed places to remove unfavorable reviews, so they are much more worthy.