You may know all this already, but just to re-enforce, here's a video on
how to fill a cartomizer. Same steps are used to "prep" the carto prior to assembling it in a carto-
tank and filling the
tank. Another trick I came up with myself is to put a wooden toothpic into the air hole to keep
juice from accidentally getting in there.
Are you using pre-punched carto's in your tank? If so, how many holes?
If not using pre-punched cartos, how are you punching your carto and how many holes? As others have said, vg juices are thick and may need more than a single hole in the carto.