Most managers can recognise a situation that merits taking decisive disciplinary action, e.g. a serious breach of health & safety, or where the attendance levels have become unacceptable.
However sub-standard performance, whereby individuals performance is below the acceptable level, is far more common and more frequently ignored. The result is that the performance impacts upon the effectiveness and morale of the team and wider organisation, leading to mediocrity.
Every manager would love to oversee a team of stars but they rarely have this luxury. However, with the right skill set and an attitude towards making a real difference, they can certainly improve the performance of the team they supervise.