In Library, drag the extra photos from B to A etc. You really need them in one collection.
In the filmstrip, notice the collection name. It's a "Recent/Favorite Sources" drop down list, and was probably what you used before.
As a tip, rather than separate collections for the sections of the book, consider using stacks. You can stack photos in the collection, and move those stacks around. So in your case, start with A, stack all those photos, drag the photos from B into A and stack this group of photos.
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+ one collection.
You must also create a "Saved Book Collection" and only ever work the book design from that.
If you set the Book Collection sort order to 'Custom', then adding images or combining other collections will help to keep the images in their 'theme' group. (any other sort will re-arrange the order of images)
Do your custom sort in the Book Collection in Grid view before adding images to the pages (manually or auto-fill). Custom sort allows you to drag&drop single or multiple images anywhere in the collection sort order.
"stack this group" also a great hint- thanks John B. (a hint I had never considered!)
Suggest also to occasionally make a 'Duplicate' copy of the book in the collection panel. It gives you a step to return to if mistakes happen (like clicking the "Clear Book" button.) Ctrl-Z ?