I'm not familiar with that motor shield or the library you're using to control it. Does it come with example sketches showing how to use it? If so, I suggest you test your setup with the standard examples before you try creating your own version.
I'm not familiar with that motor shield or the library you're using to control it. Does it come with example sketches showing how to use it? If so, I suggest you test your setup with the standard examples before you try creating your own version.