For Melasma, why does hydroquinone such as obagi or tri-luma work for some women and not others?

Some melasma has a hormonal basis so until you correct that problem, there is no longterm solution for melasma. Best, Dr. KaramanoukianLos Angeles

Melasma
can be treated, but not cured. Every patient responds differently.
Please see an expert to find what's best for you. Best, Dr. Emer

Every patient has a different response to specific product types and formulations.Some skin types are less prone to respond to topical treatment than others. 
Melasma needs long term commitment with skin care and avoidance of skin factors such as sun exposure (Spectrase Sun Block) which has wide UV spectrum and SPF 30+ protection. 
Daily good habits with topicals may well help you chieve your goals with a good skin system like the Brightening Kit from KareSkin (see link). You may want to start with Melarase and Pigmentation Correction Complex. Scler-X from another vendor is for PIH relief (see link). 
Melarase AM is non hydroquinone based and allowed for use in the EU and UK.Clear + Brilliant laser can be added once you start your pretreatment with topicals. 
Best
H Karamanoukian MD FACS

Melasma can manifest superficially in the skin and this is where creams such as Tri-Luma or systems such as Obagi Nuderm are most effective. Melasma slightly deeper in the skin responds to Chemical Peels, Fraxel Laser, or Pico Lasers. There are Melasma subtypes that are very resistant to treatment. Always be sure to protect your skin with sun exposure and discuss all triggers and a stepwise approach to treatment with your dermatologist.

Not all cases of Melasma are the same.  Some require more aggressive treatment then hydroquinones, especially if they were made worse by laser treatments such as through IPL or Fraxel.  Cosmelan or chemical peels may be needed.  Please consult a specialist in melasma.  Best, Dr. Green