It could be alot of things.
My carto tanks are pretty small ones (Boge F17 size)so I can get 5-6 refills on a carto. I have had some cartos I didn't get punched adequately burn quick though. I have also had some cartos be not great right off the bat. One thing you might pay attention to is repriming the cartomizer on each tank of juice. Burned one carto by vaping the tank dry and forgetting to reprime.