There's no better motivater than having an upset customer in your presence. I have camped outside the office of someone until a situation was resolved.
My wife got pissed at their lack of response. She took our two boys to their office, sat down, and instructed them to feel free to touch and play with everything (tile, wood samples, etc.) Salesperson notices and approaches “can I help you?” She explained the situation and the salesperson said no problem we will send someone next week.
Unsatisfied with this answer- we had already experienced many weeks of “soon” - she said ok no problem, we will just sit here until someone shows up at our house to finish the job.
Sure enough a worker showed up later that day and installed the three punch list items we had identified. Worked like a charm!
Eventually, he got what he wanted to get : some 30k€ of construction work that weren’t estimated up front and that they tried to make him pay once back to the wall. And as a plus, the company’s boss took directly the management of the project.
My writeup about this "project": https://blog.healthchecks.io/2020/09/about-tracking-cookies-...