The vapor tends to fade when they are about 1/2 to 2/3 empty. You can put the caps on, put it in a bag battery end down, and give it a spin (search youtube for "Taryn Spin") to bring the remaining fluid down to the burner. It doesn't take much and when I do that, mine vape good for another half-day and will take almost the full 2ml fill when they start getting weak again.
They also work better with thinner liquid. Straight VG just won't flow as well as PG or a blend. With straight PG in mine, I can refill almost 1.5ml when they fade the first time. I just got in some 50/50 blend and, from what I see, expect it to work pretty well too.
There's no good way to mod them, but I've had good results filling by moving the needle around the bottom of the cart for the first 1-1.25ml of fill then lift the needle a bit to put the rest higher in the carto. I consistently get 1.8 - 2ml of fluid in on every fill. After filling, blow through the mouth end into a napkin then wipe out the battery end real good with a tissue. Snap the caps back on the mouth piece and vape all day. You do waste a little juice, but the trade off is the all day sucker