Sims 2: Why Won't My Married Sims Try For a Baby?

One of the married Sims in question is actually the offspring of the two heads of the family, Jay and Kate. Jay and Kate have reached old age and want grandkids. So I had their son Reilly (the eldest) woo a potential mate and eventually marry her. Reilly and his wife Nora now live with Reilly's parents and his four other siblings (the youngest is still in diapers). But for some reason, any time Reilly and Nora have a romp in the sack, the option to "Try for a Baby" NEVER comes up. What the heck is going on? I should add that this family in is in "Castaway Stories". Help would be great!