What is the reason why there is no agglutination if positive

The antiserum has been neutralized by the soluble blood group substances or antigens in the saliva, which react with their corresponding antibody. Therefore, no free antibody is available to react with the antigens on the reagent RBCs used in the testing This negative reaction is a positive test for the presence of ABH-soluble antigens and indicates that the individual is a secretor.