Sorry for the delay, busy fixing printers last week. OK, let's tackle the magenta and black heads. Remember, the cardinal rule in troubleshooting is to change only one thing at a time and test the results.
First, we will determine if this is a signal issue, as we have don't have a single nozzle on the black head and 1/2 of the magenta. I want you to do a cable swap. See the attached pic. It's a generic example, so ignore the head color configuration. Just make sure the cables go the same same side of each head, i.e., left to left, right to right. Power down and unplug the printer. You will swap the cables on the magenta and cyan heads first, power up and then do a test print. Now power down and do the same thing with the black and yellow heads. I suspect the black head still won't fire, as based on your previous post you couldn't pull ink with the syringe. If still no black, remove the black dampers, and gently wiggle the manifold posts to make sure there are no cracked ones.
It is very important to lift the dampers straight up when removing them to insure you don't crack the manifold posts. If so, the manifold will need to be replaced. You can take one from an old head. Before installing it on the head, thoroughly clean it, flushing the manifold with acetone until clear. Also, when installing the heads, did you position them in the brown adapter as shown in the attached pic? Are the dampers and caps Roland OEM or aftermarket? If aftermarket, who is the supplier? What brand of ink are you using?
Please post the results.