As our games are played by millions of people around all around the world, this means sometimes there can be a delay in messages being delivered, due to sheer volume.
This is something we're constantly working to improve, however it does also rely on some factors which are outside our control, such as linked social media networks and internet providers.
If you're experiencing intermittent issues, we would recommend re-sending the request and if possible, check with your friend that they're receiving and responding to them.
If this does not solve your issue, please contact us and we will be happy to look into it.