None of my cities are showing any demand for commercial development. I zone them at all three levels but none ever progress. This is a recent problem after coming back to the game after a three month break - when I played before commercial zones always developed. I have even adjusted tax to low levels to try and entice shops, offices etc. Help appreciated.
Having more neighbor connections is your best bet, as well as building up means of transportation like airports and railroads.
Otherwise, I would make sure you have a balanced demand for other zones as well. Maybe you need to increase population and/or industry.