Hello
Something i have noticed when i build cities, i some times have demand for say residential and commercial at the same time, yet none of the houses upgrade to a more dense building and neither do the commercials, the only way for them to do that is to zone the entire city first.
Is there no way to force them up upgrade even if the city tile is not completely covered ?
Simply having demand isn't the only thing that will determine what develops, for example higher wealth buildings might require a nicer area before moving in. Sometimes the game stagnates, just putting in new zones or developments can kick-start other near-by developments, otherwise put some trees around, that usually makes things happen. Bear in mind that forcing growth could unbalance your city, for example if suddenly a load of $$$ Res move in but find they don't have enough jobs they can get to then you'll potentially run into problems with buildings dilapidating.
If you are having trouble getting Medium-high density, also check you have enough services to warrant such growth and note that water is a must or they will not grow. Also, until your region has the first 12,000 residents or so, pretty much no medium-high wealth buildings will grow, so if it's a new region then you need to be more patient.
Think about a city with small offices and shops, then a skyscraper holding 4-5,000 people, where exactly would these people work? This is why the simulator doesn't allow the high rises until you have a infrastructure and economy to support them and it's best to be patient.
The chance a lot will upgrade to a higher density stage depends mostly on (regional) RCI capacity, besides factors mgb mentioned. See this chart (http://sc4devotion.com/forums/index.php?topic=2994.msg92635#msg92635) for more.
I encounter this a lot when I build low wealth cities (I use CAM).
I just drop in some basketball courts and skateboard parks and the game takes off again. It seems to be a land value thing, stuff won't upgrade until the land value crosses a certain threshold.