Thank you so much, Valsimot.
This is indeed the problem. I had to replace the black ink cartridge, because the previous one had run out. From what you say, the ink inside the 'new' cartridge must've dried out. I bought it in October 2024, which I guess is a long time to keep a cartridge before you use it.
I've run the HP Smart 'Clean Printheads' function, but to no avail.
As you say, I'll need to buy another new black cartridge. Hopefully that one won't be dried out!
Thank you Valsimot.