Hidden Grottoes not cycling

I found the hidden grotto on route four just after leaving Meitchi. All the farther I have been along route 4 is the Delta Ralts in the house. I’m trying to get a Delta Sunkern from the grotto, but every time I enter it I just get an item. The first few were Level Balls, and I finally cycled to get a Great Ball just before posting this. When I look online, everything tells me I can reset the game and have the item/pokemon cycle through, but every time I reset it was the same item, no matter how I did it (soft reset, close software, save inside, save outside). Is there something I’ve missed here or is there a problem with my game?

Thanks in advance to anyone who can help me out with this.