Posting an annotated schematic showing exactly how you have wired it helps us help you. By showing all connections, power, components, ground and power sources you will save a lot of clarifying questions and time for
all of us. Be sure to note any logic wires over 10/25 inches/cm in length. Post links to technical information on the hardware items including the motors, shield, and Arduino. This should include all component values or model numbers and details of all power supplies used (which could be USB power for example). Posting the code following forum guidelines using code tags will also help. With this information we should be able to answer your question accurately. "How to get the best from this forum".
If you don't understand what a schematic is, please Google to find out. It is not the same thing as a wiring diagram. Hand-drawn is OK. Some bright, sharp photos of the circuit may also be useful.
The thing I see is many of the users do not want to draw schematics yet a large portion of there problems are in the connections, power etc. It appears they spend about a week or so with lots of questions and then maybe, give up, and some get lucky. It is amazing how many problems you find when doing a real labeled schematic including Technical links. That schematic makes them look like a pro. It appears to me that most of the time when a proper schematic is posted the solution is usually within day or so. Good schematic capture software can be gotten for the downloading. Months later a wire comes off, looking at the schematic and putting it back saves lots of headaches.
What is a schematic What Is a Schematic Diagram?
https://learn.sparkfun.com/tutorials/how-to-read-a-schematic/all
HOW to READ a Schematic: https://www.youtube.com/watch?v=9cps7Q_IrX0&t=15s
KiCad: There are many out there and you will find just like color different people like different colors, some blue, red, ..etc. CAD (Computer Aided Design) programs are like that. You will need to learn a schematic capture program that will work for you.