I am using Adafruit's Motor Shield as well. I wonder if my motor wires are not inserted into the correct terminals. I have each motor to the outer two terminals. So M1 has two wires going into it. Then I skip where it says GND, and then I connect the next two. I do the same for the other side.
I changed the delay to be 8000, and just my M3 and M4 were really going. There was less twitching (because the delay was greater), but still not moving according to the sketch. The sketch should have the motors run forward, pause, run backward, pause, release, and then restart the loop over again, correct?
As I look at the Serial Monitor, it shows the correct "tick", "tock", and "tack" for the respective FORWARD, BACKWARD, and RELEASE. I hear it make a ticking noise when it is trying to move, but other than that I get nothing. Could I have made an error in the build of the Motor Shield? Did you solder any capacitor to your motors? Is it the # of KHz that I set to each motor? I read that M3 and M4 can only get 1KHz.
Thanks for your help, Pauly.