Been having similar issues.
First possibility Airleak. Could be losing prime while it sits and when it is used again the black may have traveled backwards into the line and no black is coming out right away. If you print several of the same black blocks in a row and it seems to be getting better as it does each, that's a telltale sign.
Second possibility is hairline crack where damper plugs into adapter manifold. I have seen this on some of the videos for the "Print Doctor" machines. They were getting air bubbles repeatedly in the fluid on the K and C channels. They took a syringe with clean fluid, stuck a hose on it and lightly pumped some fluid into the head and could see fluid coming out the crack. It was hard to see with just looking at it. If you are saying someone just replaced something, it could have cracked when the dampers were put back on. That plastic gets brittle due to the eco solvent inks.
Check to make sure damper fittings are tight at the top of the damper, could be leaking air. If your printer is old like mine (7 years) may want to order some new tiny O-rings to go on all the lines at the dampers and above near where the long ink lines split into 2 lines and then go down to print head.
I have a VJ1204, and a VJ1604 which are very close to the same and since Any service call is at least $2500 I have taken the time to trouble shoot and learn to fix them myself. 99% of the time it is something with the maintenance station or capping station. That's the key to all of it. It has to have a good seal with the head when it is in parked position to be able to pull ink thru and keep it flowing to print nice.
It's best to leave the printer on when you are not using it. It does an auto clean cycle every 6 hours or so, and you can change that frequency. It's way better to pay for the ink than to pay for the repair man!