Many members access the internet and their emails only periodically and it may be few days before they reply to your message so it may just be a matter of waiting a little longer.
We have put many features in place to encourage members to do the right thing and respond to messages but I'm afraid there is nothing we can do to make them. It's also possible, if the position has been listed for a while, the home owners are busy with the applicants they already have meaning late comers get overlooked. They haven't unpublished their ad because they haven't made a decision on who they will use yet.
Unfortunately many home owners only look closely at the first few sitters that contact them and pay less attention to the others that follow. And if they 'click' with someone early on in their search, any others that come later and may be better suited get overlooked. So you should try to be one of the first to contact an owner. Experienced sitters won't wait for the daily notification of new positions, they check the website sometimes twice a day to make sure that, if there is a job posted that suites them, they are one of the first to contact the home owner.