Coaches spanning several cities seem to be of no help, as my market regularly has only 1-3 out of 240 jobs filled. Some industries even have no one working there. How to prevent this?
Although I am not sure of the exact situations, I can make a few generalizations:
Industries need goods before people head to work in them. Consumer industries, such as markets, only need one type of good to start attracting employees. Manufacturing industries, such as brewhouses, need all types of goods to start attracting employees. Note that be default, rural industries such as farms do not need employees for production (this can be set in the setting menu), as in early game is would be nearly impossible to get workers to such industries.
Most industries require workers of low class, however all passenger transport of the early era were only affordable to high class passengers, and too slow for commuting anyway (workers would usually walk). It is possible, in the line management menu, to change the prices to allow low class passengers to take stagecoaches, however that still leaves the issue of commuting time and it is not necessarily profitable.
It is always inevitable for the industry generation algorithm to create an impossible scenario with a few industries (server admins often have to make adjustments manually), however if there are problems map-wide, you may want to consider starting an new game with a higher population density.