Diagnosis
The chemical injector works on a suction principle. If it's not pulling, either the suction tube is blocked/not submerged, the inlet filter is clogged, or the internal diaphragm is stuck.
How to Fix It
First, check the simple stuff. 1) Make sure the black plastic suction tube is fully inserted into your soap bottle and the bottle has enough liquid in it. 2) The little filter on the end of that tube can get clogged with thick detergent. Remove it and rinse it clean. 3) Set the dial on the injector to the "SOAP" or high setting. It needs a strong pressure differential to work. 4) With the machine running on low pressure (use the black soap nozzle), you should see bubbles in the soap bottle. If not, the injector valve itself might need cleaning.
If That Didn't Work
The injector has a small check valve inside that can get stuck with dried detergent. You can try backflushing it: disconnect the low-pressure suction hose, and with the machine off, blow some clean water backward through the injector port from the pump side to dislodge any gunk.
Heads Up
Never use bleach or corrosive chemicals in the soap tank. They will destroy the rubber seals and diaphragm inside the injector and the pump. Use detergents made for pressure washers.