If a team member experiences a drop in performance I follow these steps.
1. Separate them and their "paired" team member from the rest of the group.
2. Fully investigate why, and find the real cause of the drop.
3. Micro manage (as a form of dis-incentive) until their high performance returns.
Average performing team members have a heavier touch than high performers. Namely they must report daily on their goals, targets and results. These 3 elements tend to force each team member to look at what is working for the high performers and adopt it as their own.
High performing team members are treated with a light touch, receiving benefits and rewards that other team members don't have, while their performance remains high. My goal is to have all team members remaining as high performers.