epub file will not convert to mobi or AZW3

Book from Mobilism in Calibre. Epub file will open normally for reading in Calibre website but will not convert for use on kindle. Message: Failed: convert book 1 of 1 (title). Subject book was downloaded in a group of 14, all the rest of which converted without problem.

Any suggestions, bearing in mind that my technical knowledge is limited?

Thanks.