If it is not flowing it is blocked - don't ignore the obvious. Maybe a bit of debris from the installer - plastic bag or piece plastic starts the blockage then everything else piles up behind. Try plunging with a rubber plunger - just create waves and it will shift the blockage. Otherwise there maybe problem with the installation - does it all run downhill etc.
not necessarily, if there is an issue with a restricted vent that could be the problem, rather than a blockage in the waste pipe itself,