What are the factors that influence dental whitening process?

A primary reason results vary significantly is because many factors influence the whitening process. Some of these factors are controllable; some are not. And they may vary from system to system and person to person. Therefore, it is difficult to make claims that will satisfy the conditions of all those wanting a whiter smile. Primary factors include:
Color to start with – it is documented that those with more of a yellowish hue have greater response to whitening than those who have more gray or brown. Intrinsic gray discoloration is commonly seen in exposure to different types of medications. The most widely known is the antibiotic, tetracycline. Brown is often seen with excessive intake of fluoride, referred to as fluorosis. Fluorosis may also create a mottling or blotchy look to the enamel and make it equally difficult to achieve optimum results.
Unique and inherent whitening potential – it is believed that we each have a level at which our teeth cannot whiten any more.
Clean teeth – it is advised to have your teeth professionally cleaned to remove all deposits, as well as surface stain that may interfere with the whitening agent being in direct contact with the tooth.
Percentage of whitening agent – meaning the higher the concentration, the faster the results. However, the higher concentration can also result in a greater risk of sensitivity. Therefore, the percentage of whitening agent should be determined based on: how sensitive your teeth are to begin with; if you have any root surfaces exposed; the delivery system you choose and over what period you want results.  In-store or over-the-counter systems have a much lower whitening agent percentage, so it takes a little longer and may require more frequent touch-up. In-office systems provide a higher percentage, the alternative of using carbamide peroxide (teeth whitening machine for sale) and a more controlled application.
Duration of time – the whitening agent is in direct contact with the tooth surface. This is delivery-system sensitive as well. And it is related to the ratio agent. Each system requires a different timeline and should be followed accordingly. The key is to maintain direct contact with the entire tooth to provide even and precise whitening results.
Compliance – is easy when you choose a system based on your needs and concerns; when you use the system as directed and when you minimize risks. It is recommended to use an agent within neutral pH range. This is safer for the enamel and reduces sensitivity. Additionally, pay attention to all the factors that influence whitening and consult a dental professional with any concerns.
Oral habits – can play a major role in the whitening process. Before any application, it is a good idea to thoroughly brush and floss plaque, stain and food debris from the surfaces of the teeth. Maintaining good oral hygiene will keep your smile looking brighter longer.
Avoid – foods and drinks dark in color or that are acidic (tea, coffee, sodas, wine, grape juice, chocolate, orange juice), smoking and chewing tobacco. Beware of oral home care products that contain alcohol and dyes/colors or may be high in abrasion or acidity.

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http://www.oyodental.com/blog/2016/03/04/what-are-the-factors-that-influence-dental-whitening-process/