I have built a simple robot, my first project. When the wheels are off the ground, they spin, when they are on the floor they don't. At first I thought it was weight but it seems not, I tried removing all the components and holding them in my hand(with the cables still attached) so it was just a bare frame. I have 2 DC motors and 7 AA batteries. The Pi is powered separately. I used this tutorial
Below are some pictures. I realise you can't see the wiring too well, but i did it exactly as in the tutorial except the extra batteries