Laser hair removal not effective. What could be the reason?

Hi, I have done 12 laser hair removal sessions (first YAG and then Alexandrite) and the results i see are very poor. The hair (especially in the bikini area, buttocks and knees) keep growing. After each session I see some results, as the hairs are shedding, but within the 5th week they start growing again, a bit weaker though. By the 6th week that i have my next session,i shave the areas and the next day the hair start growing thick again. I exfoliate&won't wax.What could be the reason?