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Switch the wires on pins 8 and 9 and see if the problem is in the shield or your code?
Remove the shield from your Arduino.
Use extension wires (male-female) to connet it again, but now cross these brake wires on pins 8 and 9.
See if your problem is still there.
If the problem now changes over to motor B, the problem is in your Uno or somewhere in the code.
If the problem stays at motor A, the shield has a problem.
Claim at the shop, or have a good look to see if there's a visible problem like some bad soldering.
Are you able to take measurements ?