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."
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