This happened after I replaced the carts following a visual inspection of ink levels. To that point all had been okay. These new carts were received just earlier this year but today was the first time since getting them that I'd actually used the matte black. I did just one print before the visual ink inspection.
Unplugging the printer made no difference, and I've tried taking out and reinserting the one cart multiple times. Tried taking out all the carts, waiting more than 10 mins then re installing as per paper instructions. Also tried taking the chip off and putting it back.
2 days later: Don't know why I didn't do this earlier. I had a separate set of new carts for the Piezo fluid to use when storing the printer. I just tried putting the matte black cart in and it was recognised. So looks like I have a faulty chip?