Why do i keep getting bad yeilds of nitroglycerine?

whenever i make nitro glycerine ...[kno3 and h2so4] i seem to get relatively bad yeilds as appose to what stiochiometric predicts. i use 50ml of h2so4 40g's of kno3 and about 10ml of glycerine yeild is about 6 ml i am using drain cleaner but even so it's about 90>%. can anyone see my problem and justify why