There are many important elements that make up the formula of successful team building. These can vary depending on what type of team is being built, but in nearly every case there is one element that ends up being so mission critical that a team's success hinges on its presence.
Take the case of the New York Yankees. Bill James has pointed out that, throughout the Yankees' history, their success has perfectly coincided with the periods when they have lots of left-handed power hitting. Their teams can have any variance of other strengths and weaknesses, but, when the Yankees have plenty of left-handed power hitting, they win. When they don't have it, they lose.
Chip Kelly's Philadelphia Eagles squad is in a similar situation. His team-building technique has proved to be highly successful, but it requires one specific strength that cannot be replaced. With this strength, his clubs have proved to be incredibly hard to stop, but without it his system falls apart.
This could be very bad news for the Eagles, as their 4-1 record masks the unfortunate fact that this team right now is bereft of that mission-critical element and will not keep up its winning ways unless and until it gets that back.