You probably missed a point somewhere in the Game.
Visit Seafloor Cavern in Route 128 using Dive.
You should have battled with either Archie or Maxie
Where in the Lava Poole their is the Solid Groudon
After you battle Maxie/Archie either Groudon or Kyogre will awaken and if you play OR the weather in Hoenn will be sunlight and the weather in AS will be rain
Head over to Sootpolis City and hopefully it will work
Good Luck :D