What is the duty of a team leader if the team is not doing well & fail to achieve the target?

If the team fails to achieve it's target then, in general, this is the fault of the team leader.  Consequently, the first thing the team leader should do is think about his/her own performance. 

Secondly, they should think more generally about the "wh" questions.   Why was the target missed?  When was the critical event?  What went wrong?  Who spotted the problem first?  Where in the process was there a difficulty?  Which sub-teams have performed well, and which badly, and why? 

Usually, when you truly - and mean truly - understand a problem, the solution is relatively straightforward.