I realize that this is an older thread, but perhaps someone else with a similar issue may read this later.
I would not consider myself an expert, but I do have several working tanks in my collection. When a carto tank is working effeciently, it is a wonderful vaping experience. The secret to this is how effectively the juice is delivered to the inner carto and ultimately the coil(s) vaporizing the juice.
There are several variables that affect this juice delivery. The tank itself really has minimal impact, it is merely a container holding the juice. Things that do impact the juice delivery are as follows:
The number and size of the holes in the carto. Juice needs a way to get from the reservoir or holding area to inside the carto. Vacuum within the tank and the act of pulling air/vapor from the carto tip forces juice thru the carto holes to the filling material inside the cartomizer, where it is heated by the coil. How fast the juice can do this depends upon the number and size of the holes. Too small or too few holes causes a dry hit, drying of the carto filler, or even burning of the filler. Too large or too many holes will cause too much juice to enter the carto and cause flooding or leaking of the juice.
Whether to use pre-drilled cartos or to punch your own with a carto punch tool or slot the carto with a powered dremmel tool seems to be personal preference. Having done all three myself, I'm currently back to buying pre-drilled cartos. Punching my own cartos can have the side effect of distorting the carto to the point of making it difficult to pass thru the tank's endcap holes. My short experience with slotting ended up with flooding issues. I've had decent success with pre-drilled cartos, as long as I choose the correct number of holes for the viscosity of the juices that carto/tank will use.
Viscosity or thickness of juice used. Thinner viscosity juices, such as 100% pg, required only one hole, and a small one at that. Thicker juices, such as 100% vg, may need multiple holes or larger holes to allow the thicker juice more room to pass into the carto. Thicker juices will quickly clog the polyfill in the carto, preventing juice from getting to the coil, and cartos used with these juices will need to be replaced sooner than with thinner juices.
Higher voltage, low resistance, and dual coil vaping. More heat and multiple heating coils will burn more juice, and faster. This may put additional strain on the efficiency of the juice delivery to the coils, and appropriate adjustments will need to be made.
Vaping technique. Someone who is a chain vapor will put increased demands upon the juice delivery system than someone who only vapes off and on. Especially with thicker juices, additional time to allow juices to wick into the carto will be required.