A little more info here towards the bottom of the article. SD was the last one to see the boy, not mom......
HPD defends Amber Alert delay for missing 12-year-old boy
HOUSTON Investigators waited three days to issue an Amber Alert following the disappearance of a 12-year-old boy because of conflicting stories from the boys parents, police said Tuesday.
"Until we were able to narrow down and confirm that an abduction had taken place, we couldn't issue one," said Kese Smith, an HPD spokesman.
Jonathan Foster was last seen by his stepfather around 1:45 p.m. Friday, police said. His mother reported him missing around 9:30 that night.
Its unclear why it took so long for his parents to alert the authorities, but police still waited until Monday afternoon to issue the alert.
http://www.khou.com/news/local/HPD-defends-Amber-Alert-delay-112574489.html