This is a complex problem. 1. remove, check and clean brewhead (round screen above portafilter where water comes out); it can become clogged with gunk. 2. brewhead gasket may be old or damaged, they can be purchased on eBay. 3. brewing basket may have become clogged, I don't know if running cleaner solution through it will help, may have to replace it. 4. Likeliest source of problem is coffee ground too fine and/or tamped down too hard. These are not professional level machines and cannot work with very fine grind or heavy tamping. Try a coarser grind and a light tamp with the larger brew basket. Coffee must be ground with good quality burr grinder for correct consistency. John Saxon/
See my espresso-making blog at motophoto.posterous.com