some issues using oils in shampoos...

Hi everyone! A few months back I made up some shampoos which used a base of castille soap or pure black soap liquid and then added some various essential oils and vegetable oils depending on whether I was making it for oily, normal or dry etc...I live in Korea where Koreans have quite thick hair and while the shampoos that I made and gave to my western friends all worked very well and achieved what they were meant to, the Koreans in about 3/5 of the cases said it made their hair kind of flat and dull and a little limp..It was odd to see such differing results..and i was wondering if anyone had any light or tips to shed on this and ways to tweak things next time to remedy that? thanks for reading! Membal...