The Engagement Program has no engagement score more than 72 hours after the Engagement Program cast.
First, make sure that is has been more than 72 hours since the Engagement Program cast, since it takes some time for the engagement score to update.
If you are using child programs in the Engagement Program stream, check the individual programs to see if leads have achieved a Success status in those programs. When programs are used as content in streams, the engagement score is based on program membership and success status, not email interaction (clicks, opens, unsubscribes). Also, if there are any campaigns in the child program that contain wait steps, the Engagement Score will not populate until the campaign is complete.