Variable product only allowing 3 attribute/variations

I am using WooCommerce with Storefront

When I try to create a variable product with four variations the product is created and apparently all variations are created as well. (For now, I have four sizes, two colours and two yes/no options)

You can see the sample product at http://sailargyll.co.uk/artandsea/product/celtic-circle/

When I try to create a product I can only select three options. As soon as I select the fourth it says’product not available’.

eg:
Size: 60×60
Colour: Red
Add Protective Layer: No
Add Reverse Facing Graphic: Yes

results in ‘Product not available’

If I change any one of the variations to ‘choose an option’ the message goes away, but of course I cannot add the product to the basket until I select the fourth variation

When I created the variations it created 32, which seems like the correct number

colour 1 in 4 sizes with four combinations of Yes/Yes Yes/No No/Yes No/No is 16 variations, times two for the second colour is 32 variations, so why is this not working?

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### WordPress Environment ###

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