If you are not sure why people aren't getting back to you, it might be due to the content of your message and therefore it may be beneficial to try another approach.
Try to be more creative and use something that will catch a user's attention. For example, rather than copying and pasting the same message to everyone you like, read their profile first, see what their interests are and send an appropriate message.
Additionally, don't forget to add pictures of yourself of the best possible quality.