Unless you just need the estates or manors for prosperity rating, sometimes they just aren't worth building. By looking at the graph you can also anticipate when you're going to need more employees and start planning housing blocks accordingly. As long as you have a stream of new people coming in you can see that you're getting plenty of young people by checking with the census advisor. As Mouse suggested, as people age, they quit working. There's several things besides a lack of housing that can cause worker shortages, as pointed out. Since you're financially sound, do you have any industries you could turn off for a while, just until you get your employment base built back up? If you right-click on it, you'll get the message that it would evolve if it had more room to expand into and that's just what you want to see-until you're ready for it to evolve.Tut-it sounds like you're doing pretty good at city management, aside from your employment problems.

A good way to block the evolution, since houses have to go up to 3x3 for scribes, is to place statues near a couple of corners of the housing you don't want evolving.

If you do want to build some of the better housing, upgrade one or two at a time and pay close attention to how many people you lose from your workforce as they evolve. I think it's lots of fun to build a huge palatial estate block, but in some cases there's not room for them and enough workers to keep a good city going. Tut-it sounds like you're doing pretty good at city management, aside from your employment problems.

Caesar 3 employee shortage late game