Soaping and Rinsing the Toilet
Let's jump from the toilet to the kitchen sink for a minute. Have you ever had a kitchen dish sponge develop an odor after just a couple of uses? It can happen when we've left it sitting in a pool of water and soap, and some blame the smell on certain brands of dish soap, which somehow speed up kitchen-sponge odor. Whatever the reason for this rapid smelliness, the same happens in the bathroom, but without the sponge: Odors start or come back very quickly even after a good cleaning.
One key might be in the products and amount of water used. Products only work as well as they're used and rinsed. Spraying something on without thoroughly scrubbing and rinsing can leave a layer of gummy "cleaner" that merely looks clean but's actually attracting more mold and keeping old bacteria hanging on with a vengeance.
Unfortunately, some of the stronger and more tacky-feeling cleansers just sit on the bowl, inside and out, and the surfaces never get clean. Cleaners with fewer ingredients, such as more natural or homemade products with citrus, vinegar or baking soda compounds, may be worth a try. Liquid castile soap is another option. Generally, these go on thinner and may require a little more elbow-grease at the start of using them, but over time they prevent malodorous buildup.