During the course of a gonococci infection, our bodies raise antibodies to gonococci pili. To avoid these antibodies, gonococci have evolved ways of genetically altering their make-up to switch from a pili+ (has pili) to a pili- (no pili) form, then back to a different pili+ form.
Scientists speculate that gonococci have over 10,000,000 different antigentic varieties of pili that can expressed, which is comparable our antibody diversity of our immune system.
So, during a gonococci infection, there are always a certain number of cells that have a different pilus structure and hence cannot be killed by opsonization (attack by white blood cells)