What can you do as a team leader to motivate an unmotivated lazy member of your team?

Challenge them and tell them that their life depends on it. :)  an ultimatum with good results.

A one on one session and a skip level meeting can be arranged to find out the exact issue instead of having perceptions of the individual.

The best  thing is to find Out what motivates the person..listen to them and try to build a good communication platform with such