Posterization implies that dark shades of ink may have been mistakenly placed where lighter shades of ink are intended. Posterization is usually a reversal of tone.
Possibly, it may be that you need to produce ICC profiles. The InkThrift are not color matched as are ConeColor.
But, if as you say, all of the nozzles are printing - then please upload a jpg of your nozzle check and we will confirm whether you have carts loaded correctly with ink. At the same time if you could take a q-tip and smear a drop of ink from each cart onto a white paper and jot down the color postion adjacent - that pic will help us as well.
Let me know if you can not upload an image.