Resin cartridge or vesell

hey i have both but I find changing getting the lid of the resin vessell hard work is there a knack to it?  

I prefer the resin vessel as it lasts longer before changing. But a nightmare to take off i have a 15 litre vessel

The cartridges are 10 inch and are easy to change but i find they need changing every other week the water from the membranes is 9 ppm and I change resin at 5 ppm.

The 10 inch cartridge i bought came filled with resin but was not completely full it had around an inch and half gap would this effect the usage.

I just hold the vessel between my two feet, grab the lid firmly and turn anti clockwise.