I print daily with my WorkForce using reman cartridges from 123inkjets.com. I have had a problem once where the ink cartridge wouldnt print or should I say, printed very light. I replaced the cartridge with another one, ran the head clean, and everything worked fine.
if you are switching back to OEM and everything is fine, I would lean toward a problem with the other carts.
one other thing i found to get darker prints, try setting your black to CMYK 60/40/40/100. much darker print