I'm seeing it with the Mantra theme now, here is a link to a blog post there specifically taken from one of the add comments links.
First thing is to go through your settings and ensure you haven't turned comments off. I know this advice sounds elementary, but sometimes we change things when fiddling around the dashboard and end up breaking stuff.
The first place to look is in the dashboard at Settings -> Discussion. The third one from the top is "Allow people to post comments on new articles". Ensure that it is checked.
Next, view the edit page of an individual post, ensure that Allow comments is selected.
Sometimes themes add their own controls for comments control (it's not common and bad practice to do so). Double check Mantra's configuration pages for anything related to comments or discussion. When you get a heavily customizable theme like Mantra is, there may be something in the configuration that is messing it up.
After you've done those things, if it still doesn't work, let us know here and we'll try some other things.