Thanks for the additional information. I apologize for not responding sooner, your response came in after I left the office yesterday and I had a family medical issue to deal with this morning, so am now working from home for the rest of the day.
Based on the information you provided, you are correctly agitating cartridges and using the printer on a regular basis. The print environment humidity is important to track and maintain, as it's important to the function of your printer, and fluctuates thru out the year. In our location, it can get VERY humid in the Summer months, so we have to run the AC constantly to keep it down, but in the dry Winter months, we have to run humidifiers constantly to bring the levels up. Dry air can easily cause ink flow and clogging issues, so humidity is especially important to monitor/maintain in the dry climates/seasons. Going by recommendations for printer, inks and papers, as well as our own experience over many years of printing- we maintain humidity levels between 40-60% for the best function of our printers, inks and papers.
Our approach is to determine the cause and repair it to resolve an issue (to learn how to fix + prevent the issue if possible), an Epson tech's approach is to replace every part that could possibly cause the issue, and will not clean/repair parts or do much troubleshooting. When we first got our 7900 (which is physically made the same as the 7890/9890 series, just with two more channels) a few years ago, and I was just figuring out how it worked (still with Epson carts installed), it had all sorts of issues, and the repair guy had to come out on three separate occasions to replace the print head (twice), ink selector damper assembly, cap pump assembly (also twice), and even the main board- but it's been working great since then (with ConeColor inks). If you search online, you will find many reports of people experiencing clogging and flow related issues with the 7890/9890, 7900/9900 and 4900 models, due to all the physical changes they made from previous models- apparently there are still some bugs to work out. We also have many users of these printers, who have only ever used Epson ink, but their printers are out of warranty and seriously clogged, contact us for help cleaning their printers. We have helped many clean their printers and get them working again with PiezoFlush, though some issues are more mechanical, and can not be fixed with flush fluid.
If this was my printer, I would install flush carts in the C and LC channels, do an initial fill and let the printer sit for a few hours with PiezoFlush in the lines/dampers, then print nozzle checks and do a few more cleaning cycles to see if PiezoFlush is able to get the channels printing again. I would also clean the wiper blade, capping station, flushing box and bottom of the print head, then do 1-3 more regular cleaning cycles before printing another nozzle check to see if anything has changed. It it's improved, but still not perfect, let it sit overnight (off), then do a few more cleaning cycles and see how things look the following day. If the channels clear up and resume printing, you can eitehr reinstall your current C + LC carts and do another initial fill to fill the lines with ink again, or drain the carts and fill them with new/fresh ink if you wish (though, we haven't had problems and have been selling/using these inks for many years, so don't suspect they caused your issues). If the nozzles don't return after cleaning with PiezoFlush, I would then remove the damper assembly (depending on your technical skill/comfort level) and use our print head cleaning kit to flush the print head, which has cleaned many clogged print heads in a wide range of printer models over the years. The print head cleanint kit can only be used on a pro model print head after the dampers have been disconnected from the head to access the channels (don't try to force thru the lines and dampers).
I hope this helps. If you need to talk on the phone, I can call you tomorrow morning if you wish.
Best regards~ Dana