Pokemon-go – What direction to walk in to capture a targeted Pokemon

Borrowing from @Codestrife's answer here

The rustling leaves usually indicate that there are pokemon near that area. Going and walking around near that area gives you a higher chance to encounter Pokémon.

Keep an eye out for rustling leaves, hinting at where critters may be lurking and simply hang out in a suspected region for the increased chance of an encounter. - Source

Therefore, the reason you don't always see Pokémon in rustling grass is because there is only a higher chance to encounter them there and not a guaranteed chance of encountering them. You just have bad luck when you don't encounter them.

You need to restart the game and try again. There is a huge load on the servers as they are adding new countries to the list where Pokemon GO is available.

If restarting the game doesn't work, reinstall it. If that doesn't work, have patience and try in a few days.