I have had this problem before. Do you have any parks? The sim starts with a cap of 50,000 people that can live in any given city. The way to raise this cap is by adding parks and other amenities (the minor league park alone raises this cap by 200,000). There is a similar cap on commercial. I have had this problem twice before, and both times I sprinkled some things like basketball courts and parks around, and suddenly the unused residential zones shot up like Roman candles. Softball fields raise the cap by 7,000. Soccer fields by 5,000 or 6,000. The tennis courts are 4,000 each, and a basketball court raises by 1,000. Most of the 1x1 plops raise the cap by 250 or 750. Education structures, health structures, and fire and police structures do NOT raise this cap, even though they increase demand.