Diagnosis
The water inlet filter is clogged with debris. This starves the pump. It's the most common cause of low pressure.
How to Fix It
Turn off the washer and unplug it. Disconnect the garden hose from the machine. Find the plastic inlet filter inside the hose connection port. Use needle-nose pliers to pull it out. Rinse it clean under a faucet. Reinstall it.
If That Didn't Work
Check that your garden hose is fully turned on and not kinked. The machine needs a strong, steady water supply to build pressure.
Heads Up
Always use a clean water source. Connect the hose to the machine before turning on the water. This purges air from the pump.