It seems like you may have a bad print head. First - do a nozzle test print to show the current head condition, which I will need you to post.
You can determine this by doing the following. You are going to do a ribbon swap, but it must be done exactly as follows. Please refer to the attached pic also.
1. Power off the printer, both sub power and main power and unplug the power from the printer.
2. Remove the head cover.
3. Remove the ribbon cables from the magenta and yellow heads, at the print head side only. Carefully examine the traces on the ends of the ribbons and insure they are not damaged in any way. If there is any damage, replace the damaged ribbons, plug all the ribbons back into their correct positions, insuring they are in straight, turn on the printer and do a test draw and see if that has fixed the problem. If you are still missing magenta, proceed to Step 4.
4. Perform the power down process in Step 1. Swap the ribbons between the magenta and yellow heads. While you are doing this, refer to the attached pic. DO NOT cross over the cables on each head. Meaning the cable from left side of magenta head goes to left side of yellow head and so forth. Very important!!!
5. Power up the printer and setup sheet.
6. Do a nozzle test print. Don't worry that the position of the colors in the test print aren't correct, this is normal for this test procedure.
7. If you are are still missing magenta, and the yellow prints, you have a bad head. If you are printing magenta, but not yellow, then we need to go further in the troubleshooting process.
If you are not sure of any of the steps, please stop and ask for guidance.
Please post a pic of the nozzle test print with the ribbons swapped.
Once I have seen pics of both nozzles tests, I can continue to help you.