I once had a similar problem, with one author's books only. Except the book would appear on the e-reader until I plugged it into Calibre the next time whereupon it would disappear from the reader though Calibre said it was there. I tentatively solved the problem by side-loading the books into the Documents folder of the reader. But later on I solved the problem by deleting the books from Calibre, downloading them again but this time converting them to AZW3 instead of Mobi. You might try something similar. You don't give many particulars, so hard to advise. And of course, your problem might not be the same.
Jim
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