What I did for my HP cartridges when they get blocked, is to get a mug of water, and put it in the microwave for 60 seconds. Then drop the cartridge into it and leave it for 15mins. After than dry out the cartridge (like leave it for an hour) and try again.

Worked on my cartridge were I only got magenta out before. Afterwards it was perfect.
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Andy MK2a 60GB Amber 040103916 32mb/Light Kit MK2a 50GB Amber 030102560 32mb