Diagnosis
The chemical injection system is either not engaged or the filter is blocked. It uses a venturi, not a pump.
How to Fix It
First, ensure you have selected the 'LOW PRESSURE SOAP' setting on the spray wand. You must use the black low-pressure nozzle. Submerge the suction tube in your detergent. Run the machine and pull the trigger. If nothing draws, unplug the washer. Remove the chemical suction hose from the inlet on the pump. Check the small plastic filter screen inside for clogs. Clean it.
If That Didn't Work
The internal check valve in the injector may be stuck. This requires disassembly of the injector assembly to free it.
Heads Up
Only use pressure washer specific detergents. Never use bleach or simple green through the injector. It will damage the seals and clog the system.