LR CC Book Module - Some thumbnails disappear at some zoom settings

Hope somebody can give some tips to help me solve this.
I am using LR CC 10.1.1 on an iMac with Catalina.
I am using the Book Module to design a book, and in the Multi-Page View when I am fully zoomed in so the thumbnails are largest I can see all of the images on every page. As I zoom out so that I see more pages on the screen some of the thumbnails disappear, such that when I am fully zoomed out only 12 of the pages out of 120 have thumbnails showing, all of the others show blank white pages.
Sometimes when I zoom in and then out again more images appear, and then I do it again and they disappear again.
I have tried rebuilding Smart Previews for every image in the book but that doesn't solve it. 
LR obviously has access to the previews of the images but for some reason is choosing not to show them at some magnifications, any suggestions most welcome 🙂

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