Develop a process that adds more openness and accountability. Hold a 15 minute daily standup meeting where each developer shares what they accomplished the previous day and what they plan on accomplish that day. Spend some time creating very distinct tasks for each developer that are 2-4 hours long. Tell the developers which tasks you expect to get completed that day and not to get sidetracked. They are providing the engine power, but you are steering the boat.

Many developers get sidetracked because they are afraid to confront the fact that they don't know how to do something. Make it clear that it's okay not to know something but it's not okay to just avoid a task in front of them. Find roles for them where they can excel. Imagine being a coach of a basketball team. Some players are good shooters while others might be good at defense and rebounding. It's up to you to find these strengths and use them together. A fast developer may get excited about doing new development but hate doing things they consider grudge work. These slower developers might like doing work like testing and documentation (and actually be better at it).