Pokemon-go – Background turned black while trying to catch a rattata

Pokémon can escape from battle, but only after a set amount of failed catch attempts. At current, there is no information on how this amount is determined, nor if it is based off the CP strength or rarity of the Pokémon.

That said, it would most definitely be influenced by the catching difficulty. You can also increase the chances of catching a Pokémon in several ways:

• Using a berry from your backpack
• Using a more powerful pokéball
• Ensuring the circle that appears over the pokémon when you hold down on the pokéball is at its smallest size, when you throw it.

For more information on the "catching circle", in regards to catching difficulty and chance, see our question on "What does the size of the multi-coloured circle mean?".

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.