Strange issue with a building's roof

[Calvin Thomas]

A couple of years ago we took over a building and had to do major rehab.  The only thing left is the bricks.  Essentially everything else was replaced.  During the rainy season last year, rain would, some how, enter in the roof and one unit would have water come in from outside.  We replaced the roof again and also sealed all over the roof and seems (flat roof) to insure there is no way for water to come in.  So far so good, but if we have rain like we had last year, nearly 3 weeks of daily rain, I am concerned this could happen again.  The roofer is out of ideas and feels everything should hold fine.  However, I am a bit more skeptical.

It only effects two out of six units, but it was very unfortunate for the tenants and the literally brand new building. Plus, we covered lodging for them while the building was re-done and their unit was repaired with new sheetrock.  Cost a good 15k all in all with supplies and labor.

Any suggestions on what we should consider to do? Even I am out of options.