Thin lining, triple layer

Ladies.. I have done 7 del estrogen injections and take 4mg of vaginal Estrace daily. I've been on these meds 20 days. My lining is good quality with a triple stripe layer but it's thin at only a 5.9. My doctor thinks I just have thinner linings as my lining has never made it higher than an 8.3 on any ivf cycle. Anyone else hear of this?