Should look like the below if using the collapsed output and the comment input was clicked otherwise will look like the below without a cancel button. There's no code for conditionally hiding the post button.
Did you modify the template files or override any of them with your own template files? If so please switch back to default CB Activity template then compare your overridden files with defaults and update your custom template. If you're not overriding the template files my best guess is a CSS conflict is causing the button to hide and suggest switching to default Joomla protostar template to see if it's Joomla template CSS doing it.