I don't have access to v7 right now - it's safest to upload v6 or lower...
To form volumes the slices need to make edges that form a loop that can face. If you have a box with an open top it will slice in the Z because each cut takes in 4 sides and makes a loop that forms a face.
If the slices are done on another axis then you'll only cut through 3 sides and these 3 edges don't make a loop and therefore won't face, and therefore don't make a sliceable volume.
To fix it give the box a'lid' and it'll then slice OK on any axis.
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