Free shipping coupon not working

Hello,

The free shipping coupon isn’t working as it should.
I have added a coupon and ticked the box to say it should enable free shipping. I have also added a free shipping method.
I am using shipping classes to assign different shipping costs for different products, but i’d like to offer a code to override all these and offer free shipping via the code.
The code is recognised in the cart, but the shipping cost is not removed.

THank you for any help!

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