New to Kindle Paperwhite

[tommy still]

I was fortunate enough to receive a Paperwhite from Santa. I have bought a couple of books, with no problems, my difficulties start when I load books in the Mobi format on to the device with Calibre.
If I leave a book to go to the menu say, when I return it consistently defaults to the beginning of the book, I am probably doing something stupid. But can anyone advise me as to why this happens, and if possible how to stop it ?

Thanks in advance.