What could be the reason behind my hair growing back so fast after 13 sessions of laser removal?

I have been having laser hair removal with alexandrite for almost two years now. The bikini area is almost neat with some hair regrowing on the labia still. However, hair on the thighs grows back almost everywhere as though no follicle was eliminated, not the same thickness but the same density. I had my last session two months ago, and it's costy, I am wondering why the hair is growing back so fast after just two months while it's my 13th session, and can I space them for 4 months?