Guys, don't take any notice of anything that everyone says about correctly copying ebooks to the kindle fire hd BOOKS folder, instead of the DOCUMENTS folder. This is the solution using calibre. Before converting your ebook to MOBI, connect your kindle to usb port. On top line of calibre options, click on arrow next to "device". Click on "configure device". Read down to where it says "documents, books, kindle" (calibre will send your book to the first folder).
Delete document so that BOOKS is the first folder. Calibre will then send your ebook to the kindle BOOKS folder everytime without fail.
Say thankyou, Guys.