Google Reviews–always keeping us on our toes! I'll speculate for a few minutes… maybe one of these ideas will help you?
Google has an active algorithmic filter in place that removes reviews that violate their policies. This has been in place for at least the last few years and I imagine continues to intelligently evolve. In the past, the review filter would run periodically and take down reviews Google deemed illegitimate. If your reviewers left comments that were against Google policies (mentioned below), they may have been removed. Or it could be:
1. Your listing or location has recently become unverified with Google without any notice. If your account is inactive for about 6 months, they may consider you inactive and you'll need to verify your account again. Try logging into your account, verify you are authorized to manage the business and agree to the terms of service. Go to Manage Location and "Edit Marker Location" — wiggle the marker and Save. Clear your cache. If this didn't work…
2. Did some of your reviews have links/URLs, email addresses or phone numbers? Google removes those.
3. If your reviews appear spammy, Google will remove them. Quoted from Google,"Reviews are automatically processed to detect inappropriate content like fake reviews and spam. We may take down reviews that are flagged in order to comply with Google policies or legal obligations.Read Google policies on spammy reviews: https://support.google.com/contributionpolicy/answer/7400114?hl=en&ref_topic=7422769
4. Reviews that pull data from 3rd party sites (Yelp, Facebook) are said to be no longer a viable option on reviews.
5. If you recently moved your location, Google should move those reviews. If not, report your location/map issue to Google here:
6. The user that created the review deleted the review. Probably unlikely, but possible.
7. As profanity filters continue to evolve, the once acceptable slang word may now be deemed as profane by Google causing the review to get filtered/removed. Social and political issues in reviews is frowned upon, i.e., reviewer(s) commenting on the efficacy of Obamacare.
8. There is speculation that if Google knows the reviewer works for you, Google will remove the review because it could be considered biased. Some folks have even been reported by a competitor as a company who "pays" or offers incentives for reviews. Google "disallowed practices" include, "reviewing your own business, posting content about a current or former employment experience, or posting content about a competitor to manipulate their ratings."
9. Your location or Google My Business account received a "hard suspension" that was not reinstated.
10. A combination of all these painfully glorious things.
Good luck to you, Justin. The rules of the Google Universe are ever changing wherein these ideas will probably be outdated in the next six minutes 😉 You might keep track of your current reviews (count, spreadsheet, screenshot) so you have documentation to open a support ticket with Google to triage the issue next time.