I would try expressing after the first feed in the morning, i always found i got the most at this time, but 40ml is a great amount, remember expressing is not an indication of how much milk you have. i always started every expressing session by hand expressing till the milk was coming out then got on the pump, i found this worked much better for me. If you get 40ml for a few days that will add up to a whole feed or two in the freezer. your body should adjust after a few days to the extra 'feed' the pump causes