This usually works for me with HP cartridges - In a small shallow dish put about quarter of an inch of warm water. Stand the cartridge in the water and leave for several minutes. Stand the cartridge on a folded piece of kitchen towel for a few minutes to dry. You should see a small amount of each colour on the towel. Now do a full nozzle clean via the maintenance routine. Repeat the soak clean if necessary.
If you look at the nozzles on the base of the cartridge and can see a build up of ink you may need to use a lint free cloth and gently wipe the residue away. (A small amount of Isopropanol diluted 50% with water will assist this process)
It`s also very expensive. You also find it in those kits for cleaning CD`s as a spray already diluted. Try Poundland. I have a good local printer that I use for commercial printing and they buy isopropanol in bulk for diluting and cleaning their machines - he lets me have a medicine bottle full every so often which can last ages.
Also found in Window Cleaner or moist tissues or at a pinch use a tissue soaked in Vodka -[But not Malt -much too good for that ;-}
Thermal printer carts[HP/Canon'Lexmark and the clones] have a 'head life'- insofar that each jet has a built in heater that boils the ink to steam to puff out the droplet. These do burn our randomly after prolonged use Thus you will see little 'whites' appearing as each jet fails.
Do not under any circumstances use Window Cleaner or moist tissues or indeed any other domestic cleaning product even if it contains isopropanol. The other ingredients are more than likely to ruin the printer heads. You need pure isopropanol diluted with water if necessary. (Distilled water preferably)