Pokemon-go – Can’t detect any Pokémon

Your situation sounds a little bit unusual, but you can increase your chances of catching a Pokémon by waiting for the inner circle to be as small as possible when your Pokéball hits the Pokémon. According to the official Pokémon Go catching guide:

You have the greatest chance of capturing the Pokémon while the colored ring is at its smallest diameter. At the opportune moment, fling the Poké Ball toward the Pokémon.

There is travel time, so don't wait too long or the ring will become maximum size again. Also note that this allows you to get "great" and "excellent" throw XP bonuses, though you have to be particularly good at aiming to make your throw land inside of the ring when it is so small.

Have you uninstalled the app and reinstalled it to see if that helped anything? A reinstallation will wipe the files on the phone and get rid of any possible corrupt/broken files. The files will be fixed on redownload.