I've tried to play Pokémon Go lately but I can't select/detect any Pokémon. I see the concentric circles but no Pokémon inside! Any idea?
I can't answer for what exactly was the reason this was happening at the time this was posted, as it's been 4 months since then, but this is still a bug in the game.
Whenever I encounter this, I am able to fix it by force closing and reopening the app.
I've had the same problems. I think it's a server glitch because of all the downloads. It doesn't happen for me now, but when it did, I had to sign myself in again (unusual right?) and then it would say I have absolutely no Pokemon registered in the Pokedex. just wait it out, that's what I did. I probably didn't help, but I hope it did.
If it continues then I would recommend emailing Niantic about the issue. The game is still in it's early stages and there are a few issues with it.
If you're seeing the circles where Pokemon are supposed to be, then they have spawned and the game is just being slow to download game assets.
Give the game a good few minutes to load the Pokemon's data in its entirety. If it still refuses to load, it may be worth re-installing the game and hoping that it works better.