Google has a filter that automatically checks reviews and automatically removes ones that it thinks violate guidelines for 
Prohibited and Restricted Content. The system isn't 100% perfect so sometimes legit reviews are also caught by this filter. Google has put out content to help merchants 
understand missing & delayed reviews.
The best way to get these restored is to contact support 
here, type reviews and follow these prompts 
 type reviews > select missing reviews > next steps > bypass resources > next steps > email
If you have screenshots or usernames of the missing reviews, that would help support find and restore them. Even if you don't have that, tell them approximately how many reviews are missing and they...