1 channel missing on a DX4 head is usually ink flow but there are some rare instances where it could be electrical. See below for a workflow of how I diagnose these types of issues:
1. Check the cartridges to make sure they haven't drawn empty but failed to flag the machine as such. If the machine thinks there is still ink in the cartridge when there is not, it will pull back on the system and prevent a channel from firing.
2. Clean all maintenance parts, bottom of the head and flush waste tubes. If the maintenance parts aren't clean or the tubes are clogged, it will cause random drop out. Clean the area on the bottom of the head where the cap normally meets up and avoid the nozzle area. Ink build up and break the seal with the cap top.
3. Test the seal of the cap top by capping the head, hooking a syringe up to the cap tube directly where the pump usually connects, and then pull the syringe. You should feel resistance and then ink should flow shortly after. If you pull straight air, the cap top is most likely in need of changing. If you pull and get a ton resistance but no ink, either the head or the cap top is clogged. In your case the head probably isn't so it would most likely be the cap top. At the end of the day if the cap top is older than 1 year, replace it. Once you know for sure that the cap top is sealing and the pump tubes aren't clogged, it's time to look at the dampers.
4. If the dampers are over a year old, I change them no matter what. If they are newer, I would just change the one on the channel you are having issues with along with the o-ring. They are cheap enough that it is worth swapping it out just to rule that out. Hook a syringe to the bottom of the new damper where it normally connects to the head and pull. You should feel resistance and the clear plastic on the side should crush down. Ink may not flow immediately into the damper depending on how far back it receded and it could take many pulls to get ink to it depending on the syringe size and how far back the ink is. Once you see ink flow into the damper and fill the very bottom of it, disconnect the syringe. What should happen is the ink stays in the ink line and the damper will start to naturally fill it self to about half full but usually a little lower at first. Now you can run a cleaning cycle and the magenta should show back up. If not, something else is going on.
5. Take the damper from the left side of the magenta and swap it with the right. If the right side continues to fire and the left doesn't. You probably either have a broken manifold or an electrical issue. If the left side stops and the right side fires, the head should be fine but there is some sort of other ink flow issue happening which means you will have to start looking deeper for air leaks etc.
Also: Just make sure there are no major kinks in the ink line which could be introducing air to the system.