Please make sure that your quest for the Water Tower is complete — look through your active quests on the left side of the screen.

Also, make sure that you have a Fire Station in your city.

You should also try to stay in one area (City Center, the Snow Plateau, etc.) for a longer time (15-20 minutes or more). If you are switching from one area to another more often than once in 10-15 minutes, the fire might not break out.