On the iPhone app, when reading a question that does not have any comments from other users one is unable to add a comment.
There is only the option to "add an answer".
For example, on this question: Several items disabled on iPhone, I am only able to see this:
However, occasionally, if there is a question that already has a comment on it, I am able to add a comment. I am unable to find an example at the moment.
Is this due to an unknown site mechanic or due to a lack of sufficient rep?
EDIT: I found an example of the "Add Comment" button appearing:
Strangely, it appeared on my question.