Why don't my hissers hiss?

A few months ago I bought a gravid (pregnant) female giant Madagascar hissing cockroach. About two months later she had her babies. I have now had about 40 roaches for a few months but I only heard the mother hiss once and the babies have never hissed. According to my research they hiss readily when disturbed and can hiss throughout their entire lives, so why have I only heard one hiss!?