Pokemon-go – Why can’t I purchase coins in Pokemon Go

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.

This has not been 100% confirmed but there have been many complaints that it gets harder to catch lower cp pokemon the higher trainer level you are. I personally have experienced this multiple times with low level weedles and caterpies.

For instance, Riggnaros says, once you reach a level in the "mid 20s," low-powered Pokémon you encounter in the game start to "have an abnormally high chance to evade capture." Source

That's a quote taken from a person who had hit level 30 days ago.

There is definitely a direct correlation between high CP and difficulty of capture as pokemon with higher CP will have darker colored rings and evolved pokemon will also be harder to capture.

Source: Personal Experience