If your contacts' numbers are shown in place of their names, you may need to reset the WhatsApp sync with your contacts. Here is how:

Open Settings (available in your App Drawer).
Go to Account > WhatsApp.
In the new screen, tap on WhatsApp again.
Press the Menu Button and choose Remove account.
Go back to WhatsApp > Contacts > Menu Button > Refresh.
If the above steps do not work, it is possible that your security application may be preventing WhatsApp from functioning properly.

To ensure WhatsApp functions properly on your device:

Go to the Security Application > Permission > Manage App Permissions > Apps tab > Select WhatsApp > turn on Trust this App >
Under privacy related, make sure everything is checked. (eg. Contacts, Use camera, Use microphone, etc)