How do you motivate co-founders to commit with you on an idea?

I am currently working on an idea with my team. All of us have jobs and are in different locations(same time zones). We're are struggling with productivity because only two members are fully commited on building the product. The two of us came with up with the idea and we brought two members who shared interest and the vision of the idea. The two other guys will promise to do stuff and then not deliver on them. How do you motivate them to work without being intrusive to their time. And how do you make sure they not slack?