Are you reusing the same carts over and over? After a week or less the white stuffing can become cranky and stop wicking well, etc. I replace tired white stuffing with with blue foam (to start out you can get cheap precut pieces for 510 at strictlyejuice.com). it holds its shape, wicks as well as new white stuffing, easy to clean (take out the foam, clean with running water and pat dry with paper towel). if make it easier to do a real cart fill i.e. take out the stuffing, fill the cup 1/2 way with juice, put the foam back, push it into the juice, using an unbent paper clip reposition the foam close to even with the top of the cart, then top off with 2-3 drops.
But yeah a cart fill is certainly not going to last a super long time. I do a full fill a few times per day and between i top off and reposition the foam if needed whenever vapor production goes down, or if it tastes dryish, or if atty is running too hot.
Cartomizers typically hold double what a cart holds.