Take no shot for granted...all shots require the same concentration from the easiest to the hardest.
It's all easier said than done.
Concentration is affected adversely by many things. For instance; motivation, distractions around the table, on the table (layout), competitive situation, or even kicked out of your apartment for not paying rent wondering where your going to sleep that night. Almost everything can distract you.
Motivation sometimes keeps the concentration at a higher level and longer period of time. For instance if you just bet the months rent, every shot will have your undivided attention no matter how simple it is.
Or win the game and impress the girl of your dreams. Motivation comes in all forms and it is up to you to come up with and recognize what motivates you. For many, practice becomes more like work and it makes it hard to concentrate.
Remember the journal I wrote of? Write down what you did on those good days and bad days.
Motivation:
Sleep:
Exercise:
Thoughts before playing:
Nourishment:
Preshot routine followed:
Better yesterdays score:
Overall play:
These are just examples I threw out off the top of my head...