Check marks will appear next to each message you send. Here is what each one indicates:
In a group chat, the second check marks will appear when all participants in the group have received your message. The two blue check marks will appear when all participants in the group have read your message.
The Message Info screen shows you who has seen your message and when. Learn more about the Message Info screen on: Android | iPhone | Windows Phone | Nokia S40 | BlackBerry | Nokia S60 | BlackBerry 10
If you only see a single check mark next to your message, there is likely nothing wrong with WhatsApp or your phone. There may be several reasons why your message has been sent, but not delivered to your chat partner:
WhatsApp Support Team