Diagnosis
This is almost always caused by a worn or damaged O-ring on the quick-connect fitting where your high-pressure hose attaches to the spray gun. A bad seal there lets pressure leak out before it ever reaches the tip.
How to Fix It
1) Unplug the washer. 2) Detach the high-pressure hose from the spray gun. 3) Look inside the female connector on the gun—you'll see a small black rubber O-ring. 4) Use a small pick or tweezers to gently remove it. You can get a replacement O-ring (standard size #10 or 1/4" ID) at any hardware store. Lubricate the new one with a tiny bit of silicone grease or even water before pushing it back into its groove and reconnecting everything.
If That Didn't Work
The issue might be upstream. Check the inlet water pressure from your garden hose. If it's too low (less than 40 PSI), the pressure washer can't do its job. Try a different outdoor faucet.
Heads Up
Don't overtighten the hose connections when you reassemble. Hand-tight is perfect. Cranking down on them with a wrench can distort the plastic fittings and cause worse leaks.