I found this a bit confusing too. You have to do things in the right order. If you haven't typed anything, you can't set the pin location. If you typed something and pressed enter, it's too late.
Type what you want to say, but don't press enter. Instead, click on the pin and when you click again to place it, that will close the moment.
Best I can see for a previous comment is to copy the text, delete the comment, and paste in the text to be your next comment.
Thanks Colin. Yes, it is not the most intuitive software i've worked with. I have done a bit of the copy/delete/paste/pin process for previous comments but it's pretty tedious given I had already entered quite a few. If Adobe is listening, this is an area that needs some improvement.