Pokemon-go – Why don’t Pokemon appear unless I restart the Pokemon Go app

You have to go into Google account and remove the device you're playing on. Then sign back in and reopen app... I literally messed around for 14 hours and missed so many at Disney because of this and it worked the first time, same problem, same phone...

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.