Pokemon-go – why is the pokemon go plus not buzzing

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.

Most (if not all) of the problems listed in the question are related to the Pokemon GO servers being offline, slow to respond, or overloaded with requests.

In the meantime, nobody likes not playing the game. Fortunately, there are a few things you can do:

• Just Wait
  The easiest (and best) move to take is just to wait for the servers to restore to a normal state. You can check the server status at this handy page, or by just trying to log in to the game.
• Force Quit the App
  Sometimes, the app will get stuck in a cycle of failure and will be unable to actually do anything. You can fix this by [iOS] opening the Multitasking window and swiping up on Pokemon GO, or [Android] navigating to Settings > Apps > Pokemon GO and then touching the Force Quit button.
• [Android] Clear Caches/Data
  As a more extreme measure, you can go to Settings > Apps > Pokemon GO and tap the Clear Data button. Note that this will only delete your local data -- your Pokemon will be safe. However, you will need to log in again. Be sure you know what account you used/created for Pokemon GO if you do this.
• Reinstall the App
  If all else fails, you can just go the nuclear option and erase the entire app and start again. If you have an existing account (and you know the credentials used), your Pokemon will be at no risk and will all be there once you log in again.