Your ink channels are preforming as they are in the nozzle checks. If your VM isn't fully printing on nozzle checks, than it isn't fully printing any other way.
Did your VM cartridge ever run totally empty? is the exit chamber about 1/2 filled with ink or more, and air vent hole open for proper ink flow?
How are you printing these purge sheets- thru QTR calibration mode following our instructions, or printing a magenta image thru the regular Epson print driver? Based on what you say, I believe you are printing a magenta image thru the Epson driver, which mixes inks, and doesn't have individual channel control like QTR calibration mode to be able to print pure ink from each individual channel. We have information that explains how to print ink from individual channels using QTR calibration mode, here: http://www.inkjetmall.com/tech/content.php?144-Print-Ink-Separation-Image-thru-QTR-Calibration-Mode
You say you print infrequently- about how often do you print vs. how long does the printer sit unused? How often have you been agitating your ink cartridges? What are the humidity and temperature levels in your printing environment? Have you manually cleaned your printer's capping station, wiper blade and/or bottom of the print head? If not, I recommend you follow our instructions to manually clean your printer: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance
After reviewing your order history, I see you purchased the set of R3000 refillable carts and set of 110ml ConeColor ink bottles in Oct. 2012.
Infrequent use of a printer, in combination with using pigment ink without regular agitation, and little or no maintenance of the printer is bad for the printer health, and can actually cause more problems than regular use and proper maintenance. Magenta and yellow inks are always the thickest of the pigment ink colors (this is true with all brand pigment inks, including Epson). Pigment settles when left sitting (in both cartridges and printer's internal ink system), which is why we recommend regular agitation of cartridges and use of printers, as well as regular maintenance to keep the printer and inks working well- to get the best output results and longest life from your printer. If inks are not agitated to maintain in-suspension pigment, and a printer isn't regularly used, thick pigment that has settled to the bottom of the carts will be drawn into the printer first (ink is drawn from the bottom of the carts), in combination with settled ink in the printer's internal in lines, will cause particle build up in the dampers to occur faster than with regular use.
I recommend you start by manually cleaning your printer by following our cleaning procedures, but due to age and use history, suspect you may be dealing with particles in the VM damper restricting flow in this channel, which will require dampers to be replaced (due to the fine filter, they can not be cleaned).
I hope this helps, please let me know your results and answer my questions above, so I can try to help you past this and back to happily printing.
Best regards~ Dana