Some locations like the Pathless Plain has 3 different legendaries to it and extra requirements, if you, for example, already have Virizion, going to the Pathless Plain at it's time of the day will result in island with ring but no legendary.Crescent Island appeared today, so I decided to catch Crescelia. No issues there. Then I decided to see what other areas I could soar to today. When I got to the Pathless Plain and Namelss Cavern, the rings were there, but no Pokémon battles began. I haven't caught any other Eon Flute Pokémon yet. Are you only able to find one per day? Thanks
Edit: I also found the Fabled Cave and I was able to battle Reshiram. Is it just random that some locations have a legendary and some don't each day?