Printer leaving streaks across every page

Depends a lot on whether it's inkjet or laser. Inkjet: go into the printer's maintenance or tools menu and print a nozzle check pattern. Gaps or missing lines in it mean clogged nozzles. Run the head cleaning, print the pattern again, and stop after two or three cleanings, because each one burns ink. If it's still patchy, leave it overnight and try once more, dried ink often loosens up. Laser: take the toner cartridge out and rock it side to side to spread the toner. A streak in exactly the same place on every page usually points to the drum (often part of the cartridge) being scratched or dirty. If you have a spare cartridge, swapping it in tells you straight away.