my epson stylus dx printer is printing poorly and missing lines?

The printer is printing poorly particulary when anything to be printed is black. It seems to miss a full line after 6-7 lines are fine. When doing a head clean and nozzle check the other colours seem fime exept the black. It shows gaps. I have cleaned and cleaned to no effect. All that does is empty the other ink cartraiges. The printer then says they are empty and have to be replaced. The ink isnt epson's own brand. Help its doing my head in. When I take the ink cartraiges out they dont appear empty? Is is just that ink is poorer quality make?