Getting Strong While Getting Lean - Q&A

I need your help. I am 37 year-old female about 128 lbs. with my last bodyfat at 18.7%. I lift 3 days a week, two with a trainer (who refers to me as a hard gainer). He continues my workouts @ 15 reps per set for all body parts with a decent amt of weight. I have not been able to budge my lean body mass/bodyfat for months.