Question : Problem: Colors missing even if print heads changed

G'day experts,
a few months ago, I had to change my print heads and I changed all four of them. All went well, until I missed some colors during printing. So after many cleaning runs, I finally renewed the print heads again. But I wasn't able to print more than 100 sheets of paper when the problem reappeared.
It doesn't help to do cleaning, second cleaning, jet cleaning, second run... it appears that there's almost no magenta (just a little bit of magenta), as if the print head was blocked. I also renewed the black and color cardridges, with no effect. I removed the magenta print head unit and cleaned the contacts smoothly with a lint-free cloth on both ends, but this won't help either.
So is there at least any chance to save the printer without purchasing another (expensive!) set of print heads? Manual cleaning? Instructions to do so?

TIA,
Michael

Answer : Problem: Colors missing even if print heads changed

difficult to answer; but here my guess : it can be caused by the cartridges being stored too long, or in a wrong position