You have no confidence. Your stroke is unsure and tentative and things can only get worse. You begin to change your stance, your grip, your style of play. And once in a while, on that special night, you click into your real self and shoot lights out. Everything works and you are once again drawn into the game with all the enthusiasm of a world class player.
Inconsistency has everything to do with fundamentals.
It is measured in the quality of hit on the cue ball. If you are not a master of the fundamentals, you willl never strike the cue ball the same way. Each shot will be an adventure in timing. In order to build good sound skills, skills that will serve you well, you must be able to hit the cue ball with a clean pure stroke. The cue tip must go through the cue ball on an exact plane.