Since WhatsApp doesn't have an official API, they automatically add their account info to the contact and when they do, we add a WhatsApp link in the contact info.
If it’s not working for some of your contacts, enter WhatsApp >> touch the new chat button >> in the select contact screen, touch the menu button in the app bar >> select refresh >> wait 5 minutes >> enter the relevant contact in Contacts+ >> check if you can see the link to his WhatsApp account.