[TheSOB88]

This might sound like a cop-out response, but I'm reading Switch by ChipDan Heath and I think it'd help you out. It's a new perspective on getting change to stick. One idea I really liked is "focusing on the bright spots" - figuring out what's working about the people who are performing much better than the others, and figuring out how to make this happen for the underperformers. For example, maybe you could ask the overachievers exactly what they're doing to manage their time, and compare that with the others.

It sounds like the underachievers want to be good employees, but you have to figure out how to get them in that direction.