HELP! Cannot see all of conversation

Hello - I have no idea why but when I reply to a message and then go back to my inbox - that message does not show the reply in the thread?? It only still just shows the original email. How can one follow messages like that? I constantly have to go to the sent folder to find when and what I sent???

In Conversation view, click the X> over on the right of the Subject line. See example below.