Prozone phenomenon is due to -

In an agglutination or precipitation reaction, the zone of relatively high antibody concentrations within which no reaction occurs. As the antibody concentration is lowered below the prozone, the reaction occurs. This phenomenon may be due simply to antibody excess or it may be due to blocking antibody or to nonspecific inhibitors in serum. Ref: Ananthanarayan & Parker's textbook of microbiology 9th edition pg:105