Drains test?

I have to do an air pressure drains test for a new build that is nearing completion. There are 3 pipes to test. One is failing, the others have passed so I know the bung and pump is working. I have filled the pipe with water to a rodding point in the house on the ground floor and the water was stable so I know the fault must be in the house. This pipe connects a utility sink, washing machine (anti-syphon connection) and boiler condensate pipe on the ground floor; on the first floor it connects a bathroom and ensuite. I have checked all the traps and the toilet connections and not found a problem. I also did a drains test on this pipe at first fix to avoid this problem to no avail.
How do I find the fault?