A little Girl
You have to talk to a Little Girl in Pacifidlog Town. She tells you stories of what her Grandpa told her. Randomly, one day, she will tell you a Story about Regigigas. On that day go to Island Cave and hopefully Regigigas will be there
>Once you have the three Regis, Regirock, Regice and Registeel, you can unlock Regigigas. The method to get this is rather complicated. In Pacifidlog Town, in the north-eastern house, a girl will tell you stories her grandfather told her. At random, she will tell you a part of the story about how to get Regigigas. This involves location and the other features. Take all three of the Pokémon to the Island Cave during the day, and make Regice hold a frozen item such as CasteliaCone, Snowball or Never-Melt Ice, and have Regice have a nickname, and Regigigas will challenge you.
If you already did and it still doesn't work try coming out and in again. Or you could Soft-Reset or just reset the game and see if that helps
Hope you find it :D