When the manufacturer's recommendations fail, try mine! I have had success in soaking my ink cartridges in a shallow plastic lid with just enough distilled water to cover the bottoms.

Soak for 20 minutes. To make the ink cartridges stand up with their business end slightly tilted in the water, I propped them up with ballpoint pens. After the soaking, gently dry off the wet area with a Q-tip.

Insert the ink cartridges back into the printer. Print a test page. If you're fortunate, the print quality will be much improved.