Motivation

I can think of different way to handle the situation:
- Daily goal: When you were at the point of falling behind projects, the line of finishing it seems to longer and longer. It will be helpful to help the behind-er break down tasks and make a list of daily goal. Completion small steps every day would definitely give the behind-er a boost. Along with that, the manager should give him/her regular feedback, so that he/she knows what to go forward.
- Distractions: Using root-cause solution might be the best to help co-workers who usually lose their focus. So, find out what makes him not be able to concentrate to his work by going for a lunch together and talking more about it with him. If his behavior affect the work of the team, it is necessary to bring it up and discuss in person with him.