If you think they pop at 4 volts (kGo runs around 4.1 full charge) try it at 5+. The lack of vapor could be the juice, or the carto might just be over full. On the first fill, it takes a few hits to get the right amount of juice in the carto. After the first fill, you can just add a few drops every couple hours to keep it wet.
The 1.25s pop quite a bit, and sometimes scare you. I have not had any hot juice leakage though. I like using a drip tip, but mainly for comfort. You will find that the 1.25s work great on the kGo. That set up is what got me off the analogs. They increase flavor and TH alot. Vapor is better too, after the initial fill. Plus, I am still using one of my 1.25s from two months ago (and several from then that have been cleaned). The one that has lasted two months is my bedtime vape, Casablanca (BWB).