Can't let down when pumping

Okay. So. I'm a FTM and EBF my lo. She is 8weeks. 2weeks ago she weighed 10lbs 1oz then on tuesday she was 10lbs 4oz. so in 1week she only gained 3oz. which they recommend and oz per day. I tried pumping both sides to see how much i was producing and with both sides i could only get 1 and a half oz! AND I can't seem to let down with the pump. only when she eats. so i dont know exactly how.much shes getting. any suggestions on how.anything could help?