One of the more popular questions found in the Google Places Help forum is of reviews going missing from the Google Map business listing. I see complaints all the time.

Vanessa from Google went ahead and built out a single post containing a bunch of the FAQs regarding reviews in Google Maps listings. The post can be found on Google Places Help and answers questions on technical issues with the reviews on your listing.

Here is it just in case it changes in the future:

Forum discussion at Google Places Help.