It can be extremely frustrating to see hard earned reviews disappear off Google. If this happens, the first thing to check is if your Google my Business profile is still verified.
How to check if you're still verified:
Log into Google My Business
Select your company location
Look for the green verified checkmark
If you are not verified, request verification. This usually takes 2-3 weeks.
Other causes of reviews being removed:
When this happens, it's safe to assume some of your Google My Business reviews have been revoked/removed by either the submitting customer or by the Google team.
Each customer is able to remove/delete their review at any time. Another possibility is that they have deleted their Google account all together, which would also remove every review they customer has submitted, ever.
There are a myriad of different reasons why the Google team revokes a review.
Reviewer seems to be too closely connected in some way to the company.
The identical review is left on various review sites
The review content is deemed by Googles Algorithms to sound artificial
The review was left from an IP address associated in some way to the company.
The review content was inappropriate or included links
The review was written from a physical location out of your service area
Too many reviews came in at the same time
For some reason, Google chose not to trust the reviewer, possibly a newer Gmail account
This is by no means a definitive list though. Here's a GMB help thread which lists all those reasons: https://support.google.com/business/thread/1774636?hl=en
If you're interested in learning the particulars: which reviews were removed and why, we recommend reaching out to Google My Business' support channel here: https://support.google.com/business/gethelp
We know how frustrating having hard earned reviews removed is, and sadly it will likely become exponentially harder to get reviews to stick in the future as Google and other companies fight fake reviews and devaluate aged reviews.