Depleting Breastmilk

Hello mummies.. My LO is currently 3mths, gg to 4. Initially I used to pump and bottle feed her with breastmilk. However, for the past 1 mth, i have been latching her direct. Recently when I tried to pump milk as I got back to work, my supply seemed to have dropped. I used to be able to yield average of 200ml. However, when i started pumping recently, I yield as low as 50ml. So two days back, i started her on formula if there is not enough milk for her (she still cries after being breastfed). There are times when I cldnt pump when im away from here. Baby weight is normal. Could any mummies advice on this? I was planning to give her breastmilk for as long as possible or at least 6mths. How can I increase breastmilk supply?