DIAGNOSIS The suction tube is either clogged, disconnected, or the venturi valve isn't creating suction because you're using too high a pressure tip.
FIX 1) Make sure you are using the BLACK spray tip. The soap function only works on low pressure. The other tips disable it. 2) Check that the clear plastic suction tube is firmly pushed onto the barbed fitting inside the detergent bottle and into the bottom of the bottle. 3) Mix your detergent with warm water if it's very thick. The machine can't suck up gel-like consistency. 4) With the machine running on the black tip, hold your thumb over the end of the suction tube (in the bottle). You should feel a strong suction.
IF THAT DIDN'T WORK The injector valve inside the machine where the tube connects can get clogged with dried soap. You may need to disconnect the tube from the machine and use a pin to clear the small orifice, or flush it with hot water backward.
HEADS UP Always run clean water through the soap system for a minute after you're done to clear the lines. Letting soap dry inside will guarantee a clog next time.