There are some people who shouldn't use whitening kits, or who have less success with the products. First of all, you shouldn't start any kind of teeth whitening until after the age of 16. If you have sensitive teeth or any amount of receding in your gums then you should also avoid whitening products. The same should be said if you have any crowns, caps, and tooth-colored fillings. These additions to your teeth aren't able to be whitened, so if you use whitening products then your natural teeth will appear whiter than the others.
You can try to whiten your teeth if they're darkly stained and brown, but you won't have much success. Teeth whitening products have shown to be most effective against teeth that are slightly yellow in color.