Summary: Ensure that you add the number in the international format. See the list below for country codes and examples.
Details:
The most common reason that people cannot add their contacts is because the contact's phone number is not correctly saved. Not sure how to add a friend to WhatsApp? Read this article.
For local contacts (inside of your country) all you have to do is enter the phone number as if you were calling them on the phone. Make sure to remove any leading 0's or any special calling codes.
For international contacts y ou must enter a plus sign (+) and then the country code for your contact, followed by their full phone number. For example the US number 123-4567 in area code 408 would be written as +14081234567. (1 is the country code).
Here is list of phone number examples and country codes from http://www.ipipi.com/

Special note for certain countries:
To add your friend in the UK with the phone number 07981555555, you would:

1. Leave out the leading 0's
2. Add a plus to the beginning
3. Add the country code.
4. In the end, the number would be +447981555555.

All Argentinian cell phone numbers (+54) should have a 9 between the country code and area code, like this: +54 9 xxx xxx xxxx
Mexican numbers need to have a 1 after +52, even if they are Nextel numbers.
If you are still having trouble finding your contact, contact us with your number, and your contact's number. Remember to write it exactly as you have it saved in your phone.

Cheers,
WhatsApp Support Team