How to fix "not enough customers" problem with your buildings?
Commercial building reports "not enough customers", fix: too many commercial zones (I think, no way to tell other than "learn it").
Customers might not be able to reach commercial zone, traffic jams and lack of public transportation for example.
Create bus line and metro line from residential areas to your commercial areas where you get "not enough customers" icon floating above building(s). Usually a good metro line does wonders for people visiting the commercial area.