After reviewing your order history, I see you originally bought 7900 refillable carts and ConeColor ink at the end of 2011. Is this the same printer that you've been using for the past 3 yrs?
When did the two channels start having the missing center sections? I see you bought a second set of refill carts and PiezoFlush in Nov 2014, so assume this has been going on for a few months.
What happened before the channels stopped fully printing- did the printer sit unused for a while, had you been printing a lot, had carts been refilled, had any carts run totally empty, etc?
How often do you agitate the ink carts?
How often do you print vs. how long does the printer sit unused?
Have you ever replaced any parts in this printer, such as the ink elector unit (dampers), wiper blade, etc?
How did you flush your printer with PiezoFlush- do an initial fill, power clean cycles, lots of printing, etc?
Do the two channels print more nozzles after the printer has sat for a while (like in the morning), and less after a few cleaning cycles, or do cleaning cycles improve the nozzle check at all?
I have found www.myx900.com to be a helpful resource for information about these specific printer models, you might want to check it out.
Please let me know and I'll try to help.