In my opinion i would do the following steps: (IM USING SUBLIMATION INK SS21)
1)Take the dampers out put a bowl under the head and send with very low pressure the cleaning solution through the dampers connection on the printhead to Clean the Printhead (Use cleaning solution for this). When printhead is 100% clean, means the cleaning solution looks same after passing through the head.
2)Check dampers or clean them. To do this. Take all the ink catrdiges out, Open valves in service menu 1 by 1 and put cleaning solution catridge in socket you will clean for example:
if you want to clean 1st damper, you take out the correct ink catridge(ussually the first), open it's valve and start sucking with a sryinge very slow. until you can see trhough the damper, and the filter of it must be the only thing that can be with color.
If you the damper filter is very dirty you need to change dampers... you can buy them everywhere i order some in dx.com freeshipping directly from china... worked for me....
Do the same with ink to take the ink back to the printhead for start printing. """DO NOT USE FILL UP INK FUNCTION IN MIMAKI MENU, IT WILL USE A LOT OF INK AND FOR NOTHING"""
3)if all that didnt work. maybe printhead is contaminating other colors... by mixing them with a leak.