frustration making an animation!

So im trying to create my first environment. It is a city environment with trees, people, roads, buildings and cars. Things start to grow upwards to build up the environment bit by bit.

However, now, I'm trying to animate the movement of each car along the roads. I'm trying to move about ten cars at the same time and every time I try, only two of the cars move. Why?!?!

I'll post up a video once its completed rendering to show what I mean.