The exit channel of a refill cart must be primed with ink after filling with ink for the first time, before installing it into the printer, for good ink flow. When carts refilled with ink or are removed from the printer for whatever reason, exit channels must be filled with ink before reinstalling into the printer- fill exit channels with ink by either tilting forward (exit valve down) and tapping a few times, or priming with the syringe and priming tip as per our cartridge instructions. If there is air in the cartridge exit channel, air will enter the internal ink lines, and interrupt ink flow when it reaches the damper and print head.
After reviewing your order history, I see you bought a set of InkThrift Pro inks and R3000 cartridges on 2/13/13, then a set of 7890 carts this September, and more ink bottles at the end of last month. How has your R3000 InkThrift setup been working? Are the two printers in the same room? Newer printer models, such as the 4900/7900/9900 and 7890/9890 are known to have common problems, such as ink flow or clogging, as well as more extreme mechanical failures, such as the print head, pump, cleaning assembly or main board. That being said, there are some things you can do to help them stay working well and lasting as long as possible. Regular cleaning is important to maintain the machine, and it's good that you use the 7890 regularly. Humidity plays a huge roll in printer function and ink flow. As per the Epson printer user guide, we recommend monitoring humidity and temperature in the printing environment, and maintain levels between 40-60% for humidity, and temps between 59-77F. We have helpful cleaning and maintenance tips, which you can read/follow here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
I hope this helps. Please keep me posted and let me know if you have further questions, or there's anything else I can help you with.
Best regards~ Dana