New Construction doesn't always work out too good

After being open a little over 30 days, we have had sewage back-ups in all restrooms and the custodial closet on 4 different occasions in 3 weeks. Each time, the pipes have been snaked. Obviously this isn't solving the problem. Each and every sewage back-up has been blamed on the staff. They keep saying the back-up originated in the staff restroom. Funny, the staff restroom or any other restroom didn't back-up prior to the branch opening to the public. The staff worked in the branch and used all restrooms from 1 month prior to the branch opening. My claims of there being a bigger issue were not acknowledged until the fourth time there was a sewage back-up. We even had two in one week. They set up a date for the plumbing contractor to use a camera in the plumbing system and no one showed up. I think the plumbing contractor is trying to get out of his liability for the issues that keep occurring.