I find that if any link in the chain falters even a bit, often your entire economy will fall apart. Assuming you have covered the basics as Dario mentioned (roads, enough workers, Inns, etc) make sure your production chains are laid out well. Sticking with bread production, I usually create two farms at 15 spaces a piece to supply one mill. The mill then supplies one Baker (you can sometimes stretch this to two but it depends on a lot of other things). I will usually allow the farm to generate at least a small amount of wheat before building the mill, and likewise for the baker. Not that you have to, but i find if you have enough to stock the production facility right off the bat upon its completion it results in less dead time.

Some other things to watch out for... Large armies eat a lot... Make sure your warehouses are setup to accept whats being produced. If you block Bread for example, your entire production line will cease when the inn is full. Make a Plaza in front of your stockpile and Inn. Usually a three lane road for me, the less one way roads you have the better. Massively speeds up all industries to not have your peasants spend 10 minutes excusing themselves to one another as they pass by on a single path.