Developed Vitiligo, Applied Melanocyl Cream Tube, Skin Sun Burnt. Why Is This Happening?

sir , i am 20 years old , i developed vitiligo at the age of thirteen and was advised by a doctor to use melanocyl cream tube . That worked and i recovered from in all the parts excepts at lower part of leg after using for three years continuously. Then due to some reason i stopped using this medicine for next four years and after that now i started again ,then only using for last two days once a day and sitting in sun after its application , my skin sunburnt intensely. Sir, i used to do the same thing earlier but this haapens now, so i dont understand why this has happed now???