If it is a sigle colour giving grief try this.
Remove the offending cartridge [or is this model a single cartridge?]
Get hold of a thin tub - a drinking straw.
suck up into the straw some vodka- hold the vodka in the straw fit it over the 'feed spike' in the base of then chamber and blow. do this couple of times.
I have moved on to a small syringe and piece of fine tube and use window cleaner instead ,and then pump
back and forth.