Motor Shield V1 and Stepper motor problem

Hi folks,

Let me describe what I’m doing, and any help is greatly appreciated.

I am building a camera slider for timelapse photography.

I have the following setup: pic attached

The motor:


The motor shield:

seeedstudio Motor_Shield_V1.0

The shield is plugged directly into a Freetronics Eleven, the 4 wire motor is wired to the shield, and the whole lot is powered by a 12V 7.2A battery.

Currently my Eleven code requests 1 rev CW and 1 rev CCW. Just to measure the distance travelled on the rail for 1 rev.

But…it does about 5 or 6 revs, starts to slow down and then stops.

I disconnect the battery, wait a minute, and then the same happens.

Not sure where I have gone wrong here, any suggestions would be great.