There are a couple of reasons why borrowers may not reply.

1. They may not be a premium member. If they don't have a crown on their profile then they haven't paid their annual subscription. As soon as they do this, they will be able to respond to your message. 
2. Check their activity scale on their profile before you message. If this is low then they are unlikely to respond. Try messaging borrowers who are more active on their account.

If you are continuing to have difficulty, then get in touch and we are always happy to help out.