This may or may not work. I use just a little bit of "push down" pressure on the drip tip (on or off, but off is easier), and use the palm of my hand to loosen the carto out of the device. It doesn't take a lot of pressure for the carto to push into the flesh of your palm, and you can use that pressure to unscrew the carto. These things are hard to type and describe. But I've never had a problem with unscrewing a tank from a device. Maybe don't use your fingers on the actual tank. Take off the drip tip and try the actual carto itself.
