My main city is having some difficulties... One day when I was growing my city, in just a second I lost about 300,000 high and medium wealth residents. When trying to figure out what was wrong, I noticed that all the residential buildings I querried had low education levels. I have no idea why, I have high schools, museums, libraries, elementary schools everywhere as well as an university and 2 colleges. Previously all my residential buildings showed up as high education. The city polls, education graph and data view all indicate high education levels.
In any case, my R$$$ sims never really came back and only built residences after I built a park next to the lot and the buildings were soon abandonned. R$$ sims slowly came back after I exited and came back to the city. R$ sims occupied any buildings still left abandonned.
As for the demand problems, my main city would always have negative CS$$ and CS$$$ demand and often have negative industrial, C$, CO$$, R$$ and R$$$ demand and fairly low R$ and CO$$$ even when they were sky high in neighbouring cities. To test just how sky high, I decided to try to use it up in the suburbs and was able to build city after city with just one occupant type (ex HT, didn't do that with Res), so in some cases demand was probably around 500,000+ in neighbouring cities but negative in the main one.
Before you ask, my main city has the lowest tax rates in the whole region.