Hi AditiS,
I understand you have an Officejet 8600 that is currently not printing magenta, despite your changing the cartridge twice already. I'm sorry to hear this has been happening, but I will certainly do my best to assist you.
I would recommend that first you have a look through this Print Quality document for your printer, as it should be able to help get the ink flowing again. Also ensure your printer is plugged directly to a wall outlet so that you can be certain the printer has enough power to function properly.
If this doesn't resolve the issue, I would recommend calling HP Tech Support for your area and see if they have any in warranty replacement options for you or what they have in their trade in and trade up options.
Hope this helps, have a great day!!