If a user is not receiving your messages, they may not be sending correctly. You can still write messages when you have a bad internet connection, but they won't be sent until you re-establish a good internet connection. Messages will remain in a queue and appear with the clock icon. They will be sent automatically when you re-establish a good internet connection.
If your messages are sending correctly, but the other user is not receiving them (you can only see one tick mark (✓)), it may be that they do not have push notifications activated or that they've deleted the application.
To resolve this issue, we recommend that you try contacting another user to check whether they receive your messages or try writing to this particular user at another time, in case they don't currently have a good internet connection.