Piotrbov: Hi.
I've got a problem. In one of missions I have 3800 population, but only about 900 workers. Why? How can I change this?
Have you been playing a really long time in that mission? People age, new immigrants are young families at working age, the proportions can change massively over a couple of decades.
Either expand, throw in another housing block if you can, or kick out an existing one. Delete the water supply, wait for people to leave the map when their houses devolve, then rebuild the well attracting fresh young settlers. Not having perfect hygiene rating also helps, they die younger that way, but its not a quick solution.
Or do you have too many nobles? They don't work either, so make sure you control which houses are allowed to become manors.