I just checked and you have 13 reviews in total. With these numbers, one single review will do a lot of damage. Sadly, there is very little that can be done about the issue of students getting a refund and leaving a bad review. The topic has been raised before. It's just part of the game. However, the good news is that this group is a tiny fraction of the Udemy student base and as such, as your number of reviews grows, you will find that the impact they have on your rating is marginal at best.
As i know send personalized emails or messages to students who have enrolled but appear inactive in the course. Ask for their feedback and inquire about their experience and any issues they may have encountered. This proactive communication may help address their concerns and prevent negative ratings.