First, fold a tech cleaning cloth (so it's not much bigger than the printhead), soak it in cleaning fluid, and place it on capstan, and leaving the printhead on top of the cloth for a minimum 12 hours. This is called "soaking a printhead".
Another way is to get 1 bag of cleaning fluid and do multiple normal or short charges. See the PDF I have attached from the Maintenance Manual. It's gonna use a bunch of ink, so it may be cheaper to get 4 bags and it to ALL 4 colors. If it doesn't clear up your problem and you tried soaking the head before all this, I'd consider needing a new printhead.