I think you are wrong about the exit pipe size. 1 1/2" is huge and would not normaly be on a water heater servicing a 6 unit building.
If the tanks are old and they used galvanized dielectric couplings to connect to the tanks you may have one that rusted to a small enough ID that some debris clogged it. Other than that we would have to be there to diagnose further.
If the plumber is competent he will find the problem eventually after eliminating all the possibilities one by one.