May I ask why you convert images to 20% dot gain for printing with Piezography and QTR? As per our manuals, the recommended Piezography and QTR workflow uses gamma 2.2, and our recommended workflow should always be followed for the best results.
There are three things main things that could cause the output shift you are experiencing:
A) if one or more cartridges were accidentally mis-filled with the wrong shade ink
B) if one or more carts were accidentally installed in the wrong position in the printer
C) settled pigment in the carts and ink lines can effect the output density and tone
To check these things to and fix your problem to resume printing:
A) Think back to any cartridges that were refilled before this problem started. To check the ink in carts to determine if the ink is correct or wrong density, you will need some of the same ink tone/shade in original bottles to use as a base.
1. Get a piece of scrap matte or rag paper and some Q-tips
2. Shake the recently refilled cartridges and corresponding ink bottles to get pigment in suspension for full/accurate density
3. Let carts and bottles rest for a few moments for froth to settle, then dip the end of a clean Q-tip into the ink.
4. Wipe the wet Q-tip end evenly across the piece of paper. Label each ink smear to indicate the tone and shade, and cart or bottle.
5. Repeat step 4 with all suspected carts and the corresponding bottles, then dry the sheet of ink smears with a blower dryer on low heat for a few moments before evaluating the ink smears.
6. Examine and compare ink smears from carts vs. bottles. The tone and density should match between cart and bottle smears for the same tone/shade ink.
If you notice differences between the cart and bottle smears, that could indicate a mis-filled cart, and the cause if your output issues. If a cartridge has been mis-filled, then the best solution is to fill a fresh cartridge with correct ink, top off any other carts that are less than 1/2 full (always double checking each cart is filled and refilled with the correct ink shade). What I find helpful to avoid mis-filling headaches, is to mark the back of each cartridge with the Piezography ink shade that is to be used in that cart (so, the C cart is labeled "2", LC is labeled "3", M is labeled "4", etc...), and always triple check the ink tone and shade of the bottle matches the cart I'm filling/refiling, before pouring ink (we have a large collection of printers, cartridge systems and full range of Piezography ink bottles that I use and maintain).
B) You can easily check the shade placement by printing the ink separation image thru QTR's calibration mode, as instructed here: http://www.inkjetmall.com/tech/content.php?144-Print-Ink-Separation-Image-thru-QTR-Calibration-Mode
Dry and examine the print. Each strip is labeled with the color position, and you can mark each row with 1-7 based on visual density (1= darkest/black, 7= lightest), then compare the shade placement/color position you printed to the ink shade placement chart to check if each shade is printing in the correct position.
C) If there aren't any noticeable differences between the bottle and cartridge ink smears, then we can assume your carts were filled with the correct ink, and move forward to find the cause (and solution) of your problem.
The next thing I think of is settled pigment, which can happen whenever pigment sits still, and occurs both in the cartridges and internal ink system of the printer. If you are dealing with settled pigment, this can be solved by removing and agitating all cartridges to ensure pigment in in suspension, and refill any carts that are less than 1/2 full. Reinstall the shaken carts and wait a few minutes for ink to settle and air to rise, then do 3 Power Clean Cycles to purge settled ink from the internal lines, and get in-suspension ink from the shaken/refilled carts flowing thru the lines and to the print head. Print a nozzle check to make sure all positions are fully printing, then print a test image to check your output.
If you continue having strange output after checking and confirming all the things listed above are correct, then another thing that could effect output density is particle build up in the dampers, which will essentially filter the ink more than the filter screen inside the dampers, and effect the output density. Also, change in the environment, especially temperature and humidity, can effect printer/ink output and how paper takes the ink. We follow Epson's guidelines and recommend maintaining humidity levels between 40-60% for best function with the printer, inks and papers.
I hope this helps. Please let me know if you have questions, your results, or if there's anything else I can help you with.
Best regards~ Dana