Thanks for the additional information. That's a lot of prints in a short amount of time for a little machine. Let's consider the possible factors in your situation: Unknown printer age + history, and unknown age of carts that came with the printer. Ink flow issues could be related to an older cart that no longer provides good flow (resolved by installing a new cartridge), the printer may have build up somewhere in the ink system (cart to line connector, ink line, line to damper connector, or damper), or the printer may have build up in the micro print head channel. Based on what you said about the amount of printing nozzles staying the same at about 20%, but the printing section of nozzles has moved up on the nozzle check printout, makes me suspect the issue is not in your print head, but more likely in the damper or ink line.
My recommendation is to flush the line, and allow PiezoFlush to sit in the printer for at least 24hrs before proceeding. Before flushing the ink line, you can test the "old cartridge" factor by installing a new VLM cart with PiezoFlush (make sure the exit chamber is primed before installing into your printer), then do a few regular cleaning cycles and print a nozzle check- if it looks good, then you may just need a new VLM cartridge with ink (and avoid the whole flushing procedure).
If you do need to flush, there's something to consider: with the unknown age + history of this printer, and knowing that all the dampers/ink supply system should be replaced every few years to maintain good ink flow, do you buy one cart with PiezoFlush and print purge sheets thru QTR to flush the one channel you're currently having a problem with (knowing the other channels may also have build up that could restrict ink flow in the not so distant future), or do you get a full set of carts and PiezoFlush to flush your entire ink system at once? It may be best to try flushing the single channel you're currently having trouble with, and if PiezoFlush brings it back, then you can resume printing (I suggest installing a new VLM cart after flushing, since the age of the current cart is unknown). If the VLM nozzles don't return after flushing, then you are likely faced with replacing the ink supply system (therefore, not needing to flush all the channels of the old ink system).
Please keep me posted, and let me know if you have further questions.