You need to quote at least part of a post from the member you want to contact, or to tag the user so they are more likely to get a notification. Some members have not properly set their notifications. AlexFisher hasn't been on this site since Nov 2.
See, I quoted your post. That helps ensure you will be notified of my response to you.
To tag a member, type the @ symbol followed by the member's name and pick the member from the popup list, like this:
@Alexfisher You have a query about your machine. Please update, is it still for sale or has it been sold?