I agree that easy misses come from lack of concentration or focus, but I think one of the glaring problems that a lot of people have is that they tend to underestimate shots. Certainly there are shots that are nearly impossible to miss, and we rarely miss them when we can use whatever speed and spin we are most comfortable with, but when you have to give it a little bit more stroke or english than you are comfortable with, you need to hit softer than you are comfortable with, your bridge is slightly awkward, or if there is a lot of pressure on you, all of a sudden that "easy" shot becomes a medium or even difficult shot.
If you shoot that shot like you would an easy shot, you are likely to miss. I have to disagree with the posters that say you should treat every shot the same. Our ability to focus and concentrate is not infinite. If you focused 100% on every shot, you aren't likely to make it through a long tournament. I think there are several ways you can stop missing easy shots, in other words, improve your consistency.
1) Improve your ability to concentrate - This is actually easier said than done, and the time you invest in improving your concentration probably wont share the same dividends as the next things unless you just have horrible focus, oh look a butterfly!
2) Make hard shots easier - This is basically just improving your physical pool skills, especially in terms of alignment, aim, stroke, adjustments for english, etc. Shots some people would consider difficult, others would consider unmissable. If you practice to the point that shots you considered difficult become hangers, you will probably stop missing the shots you once considered hangers. This is basically a matter of practice and you will improve as long as you never become content with your skill.
3) Improve your ability to idenetify what makes shots difficult - This is very important and maybe the most overlooked. Because we can't focus 100% on everything all the time, being able to ration your attention depending on the shot is a skill that can improve your game with not a lot of effort. Choosing which shots you can shoot normally and which ones you need to take a second or even 3rd look at or shots that you need to be very exacting with your bridge formation and placement is not always easy or completely obvious. A lot of the time things we overlook are the things that make "easy shots" not so easy.
If people just took a little bit of time to grasp the entire situation before each shot I think they could improve their consistency greatly. This is not to say you should ever take a shot for granted, but I think it is okay not to go all out on shots that you would make 100% of the time anyways. The key to improving in this aspect is to get to the point where you never underestimate a shot. If you do that, I think you will be able to keep your focus going for long hours while still avoiding easy misses.
