Some thoughts:
- Paid advertising
- Some campaign that could have a lot of organic reach on social media
- Differentiate with different / higher end products / products that last longer / have better taste / better vape hardware
- Ask customers for reviews on google, maybe offer them something in return
- Run some crazy promo to get people in the door

It helps to think of it like this:
- How can I bring in more customers?
- How can I have customers buy more often?
- How can I increase the amount a customer spends when he buys?

In your case, it sounds like you are focusing a lot on existing customers, but not so much on new customers. Who are your customers? What do they do? Where do they hang out (online and in real life)? What kind of people are they? That should set you on a path to find more.

I would just respond with an answer to the review(s) so that any prospective reader can make up their own mind. Something along the lines of: “We revisited and reinspected the work done. Unfortunately, as explained during the on-site visit, the damage/problem has been caused by xyz. We understand this is a difficult situation for you, but we unfortunately cannot execute these repairs free of charge as requested by you. Let us know if you want us to provide you a quote for the repairs.”