Blocked channels on the Epson 4900

I bought a second hand Epson 4900 recently which looked mint without any cartridges.
The initial nozzle pattern showed 8/10 nozzles printed well.
I hav done many power cleans, nozzle cleans an even Windex to try to clear any blocked nozzles but now 4 are showing blank on the nozzle pattern and 3 or 4 of the others are very poor pattern.
I have just done another power clean after leaving a index soaked lint free cloth over night and still the same problem.
Is there a procedure to follow and also do you have the part numbers fo the damper assemble and another part you mentioned.
(Even thinking of selling it on again and looking at a newer printer)

First things first.

Can you confirm that the vent plugs (orange little guys) are OUT of your cartridges? They should be out when the carts are in the printer.

And second, did you prime these cartridges before doing your cleanings?