The process of leaving a review is done on Google's platform. While sending review requests through TrueReview has no effect on whether the review is posted or not, it can be frustrating when your reviews are not showing up.
Google does not have a clear answer as to why some reviews are not posting as a result of filtering, however below are reasons both from Google, and patterns, observations and research into customers with similar issues.
It's important to note that the status "REVIEW WEBSITE CLICKED" on your contacts page only indicates whether your customer clicked the review website or not. As Google's platform handles review submissions, this does not confirm whether the review was posted there.
Unable to leave a review
It may be as simple as the customer not having an account to leave a review. Reviewers must log in to their Google accounts before leaving a review, so if they are unable to log in or create a Google account, they may not leave a review.
From Google (Source)
Policy violations
"There are several reasons why reviews may be removed from the page. In most cases, missing reviews were removed for policy violations like spam or inappropriate content." - Source
Business not open yet
If the business you are leaving a review on has not opened yet (Opening date set to a future date) - Source
Delays due to Google Search or Google Maps merge
"You may notice that a specific review takes longer to appear on a particular platform, like Google Search or Google Maps. This is most common after profiles are merged. Delays because of a merge should be resolved within a few days." - Source
Older mobile devices
"Customers on older mobile devices and operating systems might have issues leaving reviews. To fix the problem, customers should update their operating system and Google Maps app version."
The following reasons below are based on observation, patterns and research into customers that have had similar issues. They are not official Google reasons. Refer to Google's Help Center for "Understand missing & delayed reviews"
Newly created email address
An email address that was created shortly before leaving a review may cause the Google filters to flag it.
Email not actively used
The email address is not new, however it is not actively used to leave reviews online.
Location services turned off
Location services help Google understand whether you've actually visited the business for which you are leaving a review.
Business is not yet verified
If you Google Listing is not yet verified, the reviews may not post.
Posting too many reviews in a short period of time
Excess account activity may trigger Google's review filters and flag it as suspicious.
Photos included with review
Some users have indicated adding a photo to a review has caused delays. This may be due to Google image algorithms analyzing the image to ensure it does not violate their policies.
Google Maps cache
Some users have indicated clearing the Google Maps cache has made their review show.