Players who consistently miss shots that they should make while running balls usually do so because they change their pace. In most cases, they tend to speed up and skip parts of their pre shot routine. Speeding up during a run and taking short cuts in their routine is a common mistake made by amateurs.
Another reason for missing easy shots during run is that the player starts to become too conscious of the fact that he's on a run and focuses more on that aspect, distracting himself from the shot at hand by focusing too much internally and not enough externally on the shot in front of him.
A third possibility is that a player starts to feel pressure and slows his pace down, making himself second guess himself and start to become tentative.
In all three cases, you should always be aware of your normal pace and strive for it under all circumstances. One of my mantras for myself when I used to compete is "Find your rhythm."