I have a problem with some carto's with 65VG/35PG mix. I'm still waiting on slotted cartos from china-tech, but I have an RBA, so there is no need for me to worry.
Different brands of VG will give you different consistencies. I've been on a fuzion vapor kick lately, and I have some difficulty with cartomizers due to the nature of the VG they use.
Yes, it leaks because the liquid isn't able to pass through the filler properly. This is what people refer to when they mention 'wicking.' The material and how compacted the filler of the cartomizer is will determine how well a cartomizer functions with certain liquids.