Pulley is in action, but not movement in wheels?

Hello All, 

In grade five we are building cars. Today we experienced a little bit of difficulty where the students installed thier motors fine, but once their completed the circuit using alligator clips to a battery the wheels of the car had a difficult time moving. 

This is my first time building electric cars and was wondering if anyone has done this before and is willing to comment on the blog with some feedback.

http://gradefiveelectricity2011.edublogs.org/2011/04/06/final-conne...

Thanks!