The address book application of your BlackBerry devices running BlackBerry OS 10 automatically merge contacts in certain cases. The contacts are not really missing, your BlackBerry just shows them as merged contacts.
One example is worth more than a thousand words, so let's have a look at a case where BlackBerry OS 10 would show you just one contact, even you actually have two in your address book.

If you had the following three cards in your address book, the address book application on your BlackBerry OS 10 device would just show you one contact:

1. John Doe; Mail: john.doe@example.org; Phone: +14151xxxxx
2. Acme Inc; Mail: acme-inc@example.org; Mail: john.doe@example.org; Phone: +14152xxxxx; Web: http://acme-inc.example.org
3. Sales Team; Mail: acme-sales@example.org; Mail: john.doe@example.org; Phone: +14153xxxxx

Instead of showing you all three cards, your BlackBerry OS 10 device tries to be smart (all three cards contain the email address john.doe@example.org, so your BlackBerry assumes that it's the same person), randomly picks the name from one of the three cards and merges the data from all of them into a single one, which will look something like this:

1. John Doe; Mail: john.doe@example.org; Mail: acme-inc@example.org; Phone: +14151xxxxx; Phone: +14152xxxxx; Phone: +14153xxxxx; Web: http://acme-inc.example.org

Luckily it's possible to unlink those cards. Just open a contact, tap on "Links" and then on "Unlink".

We are quite sure that this is a bug (if the name is different, those cards shouldn't get merged) and have reported it to BlackBerry - you should do so, too if you've run into this problem. :-)