We don't know that they can't follow the lead. They may have very well followed it and found that a man was up with his baby about that time.
IIRC that lead was during the time that the parents were still cooperating. And LE may have strategized that it was best not to deny the rumor about the man and the baby, allowing the family to hang their hopes on the fact that someone else was being chased. What LE does is as much strategy as it is evidence collecting. What to release and when is just as important.
And sometimes things never get released. If it had slipped out that the man was named X X there would always be some who would not believe that he had been correctly ruled out. Even if he had really been on the porch with his own little baby, or if he had been carrying stuffed teddy bear instead of a baby. Sometimes it is better to just let things alone.