Authors often feel they have had Amazon sales that are not appearing on their sales reports, and perhaps a suspicion they are being deliberately short-changed in some way. In my experience this is not the case. Here are some of the things that might explain the apparent discrepancy.
1) Make sure you are looking at the right place in the reports, after the sales has been shipped and charged.
2) If it is an e-book you cannot buy more than one copy.
3) If it is a CreateSpace paperback, those sales are reported on your Createspace account reports, not your kindle account reports.
3) If it is a gifted book the sale occurs only when the the recipient uses it, and they have the option to use it on something other than your book--in which case you are out the money and do not get the sale.
3) Friends and relatives lie more often than you would think. If they say they bought your book don't assume it is true unless you actually saw them do it.
4) Occasionally there are glitches and reports of sales are delayed. If you can document the sale occurred and have waited about 2 weeks, ask Kindle Support for assistance.
If you know if other reason why people see these issues, please let me know and I will add them to the list.