While there is no certain answer to this question, there could be any number of reasons for this. Consider purchasing a Platinum Membership to ensure that all of your recipients can read your message. A member may not be responding because they have not been active on the site for a while; perhaps they found someone or have been too busy. Another possibility is that the member may not be interested in replying.

You can tell if a member has opened your mail if the envelope icon in your Sent messages folder appears opened.