Pixma IX6550 - no magenta even with new cart and printhead

[claymore]

Magenta stopped printing on my PIXMA IX6550, so I went through the head/nozzle cleaning routine, plus a new cartridge. No effect, so I figured that there had been a printhead failure (although it was only 6 months installed). So after purchasing and installing another  genuine, brand new Canon replacement printhead, there is still the same problem. 

But with one difference, maybe one in 20 sheets, there are some streaky blocks of magenta being printed. My suspicion now is that there is some sort of intermittent failure in the electronics, the signalling to the print head causing this. However, this isn't my area of expertise and I havent come across any HOWTOs that would help in confirming (and fixing!) this. Anybody come across this particular problem before?

Alan

[claymore · Accepted solution]

Found the problem. Not the printhead or anything in the system. Weirdly, the cartridge did not have a vent hole. So it had worked for a bit, then the vacuum that was created then prevented any further ink flow. A new cart fixed the problem and I swapped back in the previous printhead. First time I've come across this manufacturing defect.