from what I know on my mimaki:
whenever you remove a print head and put it in, you've broken the vacuum seal and chances are that there's an airbubble somewhere between the printhead and the cartridge that is keeping the ink from flowing as it should.
when I swap print heads, I take a syringe and draw ink through the bottom of my damper (sometimes the ink has run back up the lines nearly to the cartridge, so i use a BIG syringe) and inject ~5-10cc through the damper's connection on the print head (0.5cc/sec, SLOWLY), just before connecting the two together.
this way, i know that there's ink all the way through the system as it should be... and I don't have to waste anymore ink doing a FillUpInk, because I know the printhead is already "primed" with ink.
