On is an interesting map because it is broken into islands, none of which have access to all the needed resources or terrain types. Of course, since you've been playing this map for a month, you must know that already. =) But I'm reminding you of this because On requires a very specific system of ferries and/or bridges for things to work smoothly. If you have perhaps turned off pop-up messages, you might not be seeing certain warnings. Is your food actually being distributed properly? Distribution is challenging on this level...
A little more information could probably help the folks here diagnose your problem more accurately. For example, when people leave, what sort of housing are they leaving -- has it devolved, and if so, to what level (i.e. rough cottages? Meager shanties?) Your advisors can often help, too...
This level did require some thoughtful planning, but once your network of transporation was in place, it would pretty much run itself. On was one of my favourite levels in the entire game, so much so that the only scenario I ever built was based on On. I was about 90%+ of the way done with it, just had add a few more pharaonic requests and other events to make it interesting, but I never released it... it took up where the official On left off, requiring you to move goods across all sorts of tributaries, inlets, and deltas...
Stebonius