I think I have discovered the problem. If you make a comment and submit that is fine. To leave the thread, if you hit the back button (the arrow on the top of the page next to the https:/) you will lose your post as you have asked it to go back to where you were before. To leave the thread click on the ‘Forums’ button at the top of the page, it takes you back into the Forums obviously but you will not lose your post. Just my observation but I could be mistaken.
