I know I complain about having trouble trying to contact DNA matches at Ancestry but I believe I've found a way that works pretty well. If your DNA match has a tree, comment on the ancestor of interest. I did this early yesterday morning and included my email address. Within 30 minutes I had 3 emails from cousins two of which had DNA tested off of that branch.
One of the biggest headaches at Ancestry is trying to contact others. There really is no reliable way. Searches of Member Directory, Message System and AncestryDNA matches are all sketchy and almost never consistent. Sometimes the only way to find someone is to pick the oddest surname they have in their database and search on that.
Why does commenting on a tree work better to get a response than trying to use the Ancestry Message system? I don't know for sure, but it may be that the email that is generated when comments are made doesn't have all the "clutter" that the other ones have. Also it helps that the comment is attached to the Ancestor of interest. I just know it has worked for me and I will continue using that when it is an option. If you try this, I'd love to hear your results.