A pendulum swings down and goes up but not to the same heigh…

A pendulum swings down and goes up but not to the same height as before.  One student says it should have been able to make it.  A second student says there is no way this is possible.  Use the idea of conservation of energy to explain why the pendulum did not return to the same height as before.  Is it ever possible to have the pendulum reach the same height?  If yes, under what conditions would allow the pendulum to reach the same height?  If no, explain why.