Diagnosis
The soap injector on these is a venturi system that requires a specific setup to work. If you don't have the right tip attached and the pressure set correctly, it won't pull soap.
How to Fix It
First, make sure you're using the black chemical injection nozzle (it's the one with the widest spray pattern). It must be clicked onto the wand. Second, the pressure control dial on the handle must be turned down to the lowest setting. High pressure bypasses the injector. With the black tip on and low pressure set, you should see soap being pulled from the bottle.
If That Didn't Work
Check the suction tube that goes into your detergent bottle. Make sure it's not kinked and the filter on the end isn't clogged. Also, ensure the selector dial on the pump itself is turned to the 'SOAP' symbol, not 'WATER'.
Heads Up
Always rinse the soap suction tube with clean water after use. Detergent left in it can harden and permanently clog the tube and injector.