Building a mousetrap car for my engineering class -- Help please

[unknown5174]

Summary:: My back wheel spins when in the air, but won't when on the ground, help!

My car has two four inch wood wheels on the front and a 12 inch wood wheel on the back, they are securely attached to the axel and I have 1/2 inch ball bearings to connect the axel to the frame. My lever arm is about 18 inches long. I have scoured the internet for an answer but I just can't seem to find why it won't spin while on the ground.

 

[berkeman]

Welcome to PF.

Can you post some pictures? Use the "Attach files" link at the bottom of the Edit window to upload JPEG or PDF files.

 

[Lnewqban]

Welcome!
Could each of the bearings be not in perfect alignment with the axle?
What if you slightly push the car forward?