There are two possible reasons.
The most likely is simply that you've used poor-quality paper. Using basic 'multi-purpose' printing paper almost invariably results in 'washed out' printing. For printing documents you should be using a good quality 'high-white' paper which is intended for use solely with inkjet printers. For printing photographs you need to use top of the range' photo printing paper (such as Kodak Ultima) and not the muck that many supermarkets sell (although 'Tesco Finest' photo paper is excellent).
The other reason could simply be that the print heads need cleaning (although blocked print heads generally result in 'banded' printing, rather than in 'weak' printing).