After a while the filler can get kinda packed down and hold on to their juice too well, which keeps more juice from absorbing, and it just runs through the center air channel. Any juice in that center hole can just run through, so even if the carto is new, drip to the outer edge drop by drop somewhat slowly allowing the juice to soak in before adding more. If the juice is starting to come out the end, that's my clue it full, and I blow the excess out through the mouth-end into a paper towel, screw it on and enjoy.
Sounds like yours might be in need of either replacing, or if you want to you can clean them...I soak mine in vodka for a day, then flush them thoroughly with hot water, dry in a 200 deg oven for 30 min, fluff up the filler a bit, and then reuse. If all goes well, they should bounce right back to almost new condition. I get about 3 days of use out of mine before I decide to clean them, but I get many cleanings out of each, and I've got cartos that have been great for weeks and weeks, even months.