Unable to send and receive messages on whatsapp?

2 days ago my whatsapp on my iPhone 4 was working fine, however the following morning I'm unable to send and receive messages from one particular contact. I'm able to send and receive messages on whatsapp from all of my other contacts. In the chat, the "last seen at… " was 2 days ago, however they were able to send and receive messages to their other contacts as well. We both have full signal, and we're both connected to the Internet. We've both turned our phones off and on and turned our Internet off and on. I have also deleted and reinstalled whatsapp but still have the same problem. When we try and send each other a message, only one tick is displayed. On the whatsapp FAQs, this means that's the message has been "delivered to the server" but not delivered to the recipients phone. On all other whatsapp chats, I get the two ticks with no problem. Finally, we both have the most recent version of whatsapp. I've also emailed whatsapp support and currently waiting for a response, but I'm not expecting a quick reply.

Added (1). And we haven't blocked each other either

There's a possibility that your partner may have blocked your number in WhatsApp. It can be intentionally or can be by mistake too. Please you both check that you have not blocked each other by mistake.

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