Pokemon-go – A Pokémon suddenly appeared in the Pokédex. Does that mean I’ll see it soon

From this reddit post I found out:

• Same Pokémon spawns for every player in the same place.
• Every player can catch it.
• Once it is caught by one of the players, other players who faced it can still catch it.
• Combat Power depends on the respective player's level.

Intensive playing of the game proved those facts. Incense might bring different results as it has only impact for the user.

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.