Many chess players have a wrong idea about opening preparation, how it should be done and what you should expect from a good opening repertoire. They think that by memorizing a few lines in some opening they would be able win a game after game easily.
They think that the cause of all their failures is the opening preparation, when in fact it is their tactical vision (or lack of it). However, on the higher level opening starts playing a much more important role. In fact, on Grandmaster’s level, it often becomes the most important part of the game.