As Deana asked, you really need to let us know which revision of the printer you have. Substantial changes were made to the ink delivery system and were released with the MP5i (aka "Rev C"). If you don't have this version, you should look into getting your printer updated to the "i" / "Rev C" version.
Also, you might want to call into tech support and have them check the lot number on your magenta ink cartridge. There is a specific batch of magenta ink that tech support suspects has been causing problems for mPower owners.
If you have the "i" version and a magenta cartridge that isn't suspected of problems, look into making sure the maintenance station doesn't have a blockage (as Deana suggested). You might also want to run a 20 or 30 second print line fill. If that doesn't clear it up you could try soaking the printheads for 30 minutes in a mixture of cleaning solution and distilled water that has been warmed in a microwave for 15 seconds - simply release the print carriage and pour the warmed liquid mixture into each of the ovals in the maintenance station until they are filled to the top and then secure the print carriage for 30 minutes. Afterwards, go into the purge line fill menu which will turn on the maintenance station pump to clear out the liquid in the ovals (just go into the menu, do not actually start filling the purge lines), then run a printhead clean and see if that clears up your problem.