Review reminder email not sending about 50% of the time

Hi,
I’ve been testing the review reminder email for a couple of days.
For me, only about 50% of the emails are sent.
I configured reminder to be sent on Complete status with no other conditions.
I tried different schedule times such as 1 second, 15 seconds, 300 seconds, 5 minutes, 20 minutes.
Some order I tested, _wcpr_review_reminder is still pending when “time” is 2 days ago.
I checked cron list and can see there’s wp_wcpr_process_cron_interval set to 5 minutes.
How can I check why some emails are sent and some are not sent? I completed about 20 orders for testing and only about 10 orders sent review reminder emails.

Thank you.