Plumbing Job?

I have a duplex rental unit that had the main sewer line that was stopped up. The plumber came out & ran the line through the pipe on the roof to the sewer drain in the yard. It took an hour & a half. They told me to call the city & have them put a clean out drain at the curb. The city came out & said that I didn't need one. They ran a camera down the main drain & the stoppage was in the middle of the house they said. The next day the tentant called the plumber with the same problem. Everything was still stopped up. Plumber came out again & it took another hour & a half. I paid the first bill while I was there on site the day before & then I get the second bill in the mail. Do I owe for the second bill if they didn't clear the line the first time & also I figured the city would have cleared it since they were there??? Why would it have been stopped up after the first attempt, plus the City coming out there and then they had to come again?