Best way to have a legit reviews on your trustpilot page regardless the rating is to invite your clients/users using the email used for truspilot, when they make reviews there is a label "verified" which means the reviews came from your legit past clients.
Thus fake reviews are easy to identify and reporting them or flag them (reviews) as abuse is the best way to remove them on your truspilot page.
We encourage clients to leave reviews, however not every client of ours leave a review
We have a rather small smm business, there are a lot of competitors so sometimes it's tough.
You should keep encouraging your customers to keep leaving a review of your services because that would help your business standout from the others if they derive maximum satisfaction from them. You already said that there are lots of competitors which is a good reason for you putting efforts to render good services to your customers so they could leave a positive review for you so that any prospective client who sees it would contact you.
I think if you follow the steps PX-Z has mentioned, it would help you a lot as your customers would easily do it for you if you attach link to your trust pilot page in the body of the mail to them for their review. It would also show the authenticity of your services.