Here's a question for you - what sort of structure are you providing to encourage your people to perform?  Do you have a mutually beneficial and understood verbal or written contract that specifies who is responsible for what, what kind of work each person is responsible to put in to be counted as a full participant, and what kind of rewards they can expect for staying the course?  

People are people, regardless of how smart they are.  They lose interest quickly, get distracted, and often after a while begin to look for reasons to put in less than their best efforts, especially when the payout for their work is seen as iffy or a long way off.  Very few are disciplined enough to work consistently for long periods of time towards a goal without some incentive along the way to continue to do so. Could this be what needs to be tweaked, or maybe has been missing altogether, in your last three efforts?