My milk supply doesn't meet my baby's needs!?

I've been nursing my 4 week old baby 14 to 17 times a day since she got home from the hospital. Each feeding takes about 45 minutes. 15 to 30 minutes later she is screaming for more. I know I'm supposed to feed at the first sign of hunger, i.e. rooting...but if I did that, she'd be eating constantly (which I sometimes let her do for several hours, switching every 5 minutes hoping it had time to generate a little more milk). Anyway, everything I've read talks about supply and demand and that after a few days of frequent feedings my supply will go up to meet her needs...but it's been 4 weeks! Also I'm taking Fenugreek, blessed thistle and domperidone. I'm pumping at night since that's when she goes the longest stretches without eating (2 to 3 hours). Sometimes when I pump, nothing comes out, other times I get 1 oz from both s (but this is after she's just finished eating...so this is good). She's gaining weight very well, 14 oz. this week! I just can't get her to go longer between feedings. When I leave her with my mom and she feeds her, she easily takes 4 oz. in one sitting, leaving me feeling like I don't have enough for her. I can pump about 2 and a half oz. total if I haven't fed her recently. HELP! All I ever do is nurse!