There's an update coming to Pokemon Go that will take some of the frustration out of trying to catch rare Pokemon.
Called the "capture bonus", the update (coming soon) will give you extra power to catch certain types of Pokemon, based on your achievements in the game so far.
The catching bonus revolves around the medals system that Trainers are awarded for capturing certain types of Pokemon. If you catch a lot of water type Pokemon, you earn the Swimmer medal, for example, in various stages.
This currently has no function, it basically just gives you a sense of achievement, but the catch bonus means that you'll have extra power to catch Pokemon of that type.
That means that if you come across a powerful Pokemon with a high CP, you'll have a greater chance of catching it, as your previous catches give you a boost.
If the Pokemon you're after is multiple types, then your catching bonus will be the average of your medal status for both those types.
It hasn't been revealed exactly when the update will land, but it should be hitting your Android and iOS devices soon.