When I am viewing the latest message in a conversation in Mail in the preview and want to reply, I hit reply and it does not include the latest message? I have to double click the latest message to open it and then reply. Am I missing a setting to fix ?

I have found that if you have highlighted some text in the message and then hit reply, the highlighted section is the only bit included in the reply. You need to click on the message in the middle list rather than the preview pain on the righthand side and it should reply normally. To check it's doing this, select a message in the middle section of the screnn then use the down arrow to drop to the next message. Hit reply, and it should reply normally. I hope this helps.