Diagnosis
The chemical injector system is either blocked or not creating the necessary vacuum. The most common culprit is a clogged filter in the suction tube or a blocked injector nozzle.
How to Fix It
First, remove the clear plastic suction tube from the bottle and the machine. There's a small filter basket on the end that goes into the soap. Rinse it thoroughly under hot water. Next, with the machine off and water supply disconnected, locate the chemical inlet on the pump (where the tube connects). There's a tiny nozzle inside that orifice. Use a thin piece of wire (a straightened paperclip works) to gently poke through it and clear any dried soap or debris.
If That Didn't Work
The dial that selects between water and soap might be malfunctioning. Turn the machine off and cycle the selector dial back and forth vigorously several times. This can sometimes free up a stuck internal valve. If soap still won't draw, the internal injector valve may need replacement.
Heads Up
Never, ever leave detergent in the suction tube or bottle for storage. Always run clean water through the soap setting for a minute after use to flush the system. Dried detergent will absolutely clog the tiny passages and ruin the injector.