Rizty, i also has same exact condition, drop from 100ml to 40ml,i have contacted breastfeeding support group and like cactus mentioned before, the counsellor advise me to pump more diligent and pump at a regular interval, say 2-3 hourly, and even including the middle of nights as our bodies make the most milk between 1-5 am and it would be essential to express at least once during that hour to keep the body producing milk.
Hope this helps.