Comment on Custom Posts Widget by SEO Dave.
Try moving it to the left or right sidebar widget area.
Try adding a simple line of text like:
Testing single posts widget
This will rule out code errors if you are trying to add broken HTML code for example: broken code could be loading, but because it’s broken you don’t see it.
Don’t forget to clear your cache if you use a plugin like W3 Total Cache and when you check the WordPress Post the widget should load on use CTRL F5 to force a browser refresh.
I use a comment subscription plugin “Subscribe to Comments Reloaded” for managing comment subscriptions.