It's difficult to tell without seeing your book.
Did you write it in English? I'm asking because the English in your question isn't very good, sorry, and it seems to go beyond the occasional typo, so if your book is in English and your friend didn't do a good job with the editing/proofreading, then that could answer your question.
Other reasons could be a poor cover, where you've put the book for sale, lack of marketing (if people don't know your book exists, then they can't buy it), your pricing is off (Too cheap? Too expensive?), the topic appeals to just a small group of people etc.
Add to that, that self-published books just don't sell very well in general, even if everything is spot on. It is really a tough market to break into.
To sell well, you need to approach this as a business man, so take a good hard look at your book, and see if there's anything you can improve. Also, if you don't know how to do this already, then study how to best market your book, how to price it right etc.
Best of luck.