The wheels arent keyed onto the axel are they?
But its more than likely what bob said.
The wheels are not locked on, just two washers and a grommet to hold them on. There is a two inch 1\4 centimeter strip engraved on the axl where the attatches.
In that groove in the axle should be a steel key way. The wheel needs to be locked to the axle shaft or it will just spin and not go anywhere.