They might be bored. They might be in over their heads and see two co-workers knocking stuff out and not want to appear like they don't know what they are doin

They might be bored. They might be in over their heads and see two co-workers knocking stuff out and not want to appear like they don't know what they are doing. Do you have regular one on ones with them to shoot the breeze about what they're working on? Find out what motivates them. Maybe they don't believe in the work going on and think the lead/manager (aka you) is just giving them busy work. Explain the broader picture of what their contributions mean to the organization. Did they get "passed over" for the role you now have? If you work in a big company, maybe they have seen where getting work done quickly doesn't get them ahead. Maybe there isn't any room for them to grow. What do they want to be when they grow up? When assigning work, ask them about their approach to the task. Ask them to ask their coworkers or bounce ideas off them. Maybe they think building VM's will help with testing for future work. You may not be their HR manager (hire, fire, raises, etc.) but talking with them about non-task related stuff may enlighten you with how to better work together.