This could be because of age,
lack of egg production, a history of STI's, problems with the uterus or the fallopian tubes, hormonal imbalances, early menopause, and having excess tissue around the reproductive organs (endometriosis).
Still, even the talented cast and skilled computer animators can't compensate for the
lack of connection the audience feels for this coop
of characters — leaving this
production a couple
of eggs short
of a full dozen