You didn't mention what printer and cartridges you are having problems with, but I guess that the cartridges are of the PGI-520/CLI-521 or PGI-525/CLI-526 type?
I have noticed more threads about these cartridges with inkflow problems than with the older transparent cartridges, especially when they are refilled using the "German" method.
Your printhead could have failed electrically, but in that case the fault often affects the PGBK or Dye nozzles in a way that shows exactly half of a printed band faulty, like a picket fence pattern in PGBK or half of a dye band missing or significantly lighter than the other half. Another symptom of electrical printhead failure is either PGBK or the dye bands missing completely.
So I think you have a magenta inkflow problem. This could be caused by either the cartridge or the fine passages in the upper part of the printhead being clogged.
Your test has shown that the printhead is not clogged in magenta. It it not recommended to pressurize the printhead, so hopefully this test has not damaged the printhead.
I recommend buying a new OEM magenta cartridge and trying printing with that. If magenta now prints OK you know that the magenta cartridge(s) is/are faulty. Purging and drying of the cartridge(s) could then bring it/them back to working order. If a new magenta cartridge doesn't print properly, probably the printhead and/or the logic board is defective.
You could also do some inkflow tests of the cartridges, comparing a working cartridge like cyan to the suspect magenta. Touch a piece of kitchen paper to the ink outlets of the two cartridges and notice how quickly the ink wicks into the paper. Also hold the cartridges over a sink and notice how hard you have to blow into the vent before a drop of ink leaves the ink outlet. I have had failed cartridges that were very difficult to blow ink out of.
See this thread for an inkflow problem I had, especially note the the posts that mention sponge displacement and an air-lock:
http://www.nifty-stuff.com/forum/viewtopic.php?id=7744
I have a suspicion, but no evidence yet, that the newer windowed and opaque cartridges are more fragile than the older transparent cartridges, and that top filling may be the least damaging way to refill the new cartridges. If I remember correctly
ThrillaMozilla reported sponge displacement problems with the opaque HP 564 XL cartridges that were refilled using the Freedom method. The cure is to tap the cartridge hard against a table with the ink ink outlet facing down and protected by a storage clip or similar.