Whatsapp: could it be that messages from some contacts are received but not from others? "My mum and her phone are driving me crazy" must probably be an entry in many personal diaries by now...

The thing is: my mum claims that she sends whatsapp messages to some of her contacts (there's a double blue tick and everything, so they've read them) and she doesn't get any replies. However, she does get replies from some other contacts. And I just tried myself: messaged her and the message arrived instantly.

Could there be a reason for whatsapp not getting messages from specific contacts only? (I just checked her phone settings and no contacts are blocked).
The double-check indicates that the message received. So, it might be possible that the recipient simply chose not to reply.

You mentioned that your mom hasn't blocked anyone. How about if your mom has been blocked? You can check for these indicators: 

- You can no longer see a contact's last seen or online in the chat window.
- You do not see updates to a contact's profile photo.
- Any messages sent to a contact who has blocked you will always show one check mark (message sent), and never show a second check mark (message delivered).
- Any calls you attempt to place will not go through.