Many times we get more mosquitoes or no effect after fogging because a lot of times it is done wrongly.
1: Wrong time. Fogging will be most effective during the time the mosquitoes just start to get active (from 5am to 7am for the dengue-carrying Aedes mosquitoes). But people would most likely complain if noisy fogging is done during that time and so the pest control people have no choice but to do it in the late morning or afternoon, where it doesn't have the desired effect.
2: Wrong method. Some less experienced personnel hold the machine wrongly, or aim the fog at the wrong places. Also, it is rumoured that some companies stinge on useful chemicals and merely use a very low concentration mixed with diesel to get the visible fog, but doesn't do much good. I remember actually reading newspaper reports about companies cheating in this way.
3: Wrong chemical. Same chemical used over and over causes immunity in any pest over some time. Ideally different chemicals from different groups should be used in rotation, but many companies use the same over and over, thus resulting in harder to kill pests.
Fogging done correctly at the correct time CAN be a great help to bring numbers down, but permanent control can only be achieved by environmental control, eliminating suitable breeding sites to prevent their reproduction.