Asset Browser - material preview not showing (here), why?

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I made some material assets for the Asset Browser.

However, only two show up in the preview. When I drag that material onto an object it works, but I’d like to see the preview so I have a better understanding of the material I am dragging.

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I had to go through each material and clear asset, then click on to different previews and save that .blend file, then the preview showed up.

Is there a script I wonder which will take each material in a batch of objects I select and then add them all to assets?