Make sure you are in the 'My Responses' tab.
Viewing A Response
To view a response you've sent or continue with an existing conversation you'll need to click into that exchange. You can do this by selecting the "My Responses" section of your account and clicking 'View' option next to the relevant response.
This will bring up the following screen. From this conversation screen, you'll be able to see all messages sent to and from this particular buyer.