It happened already many times: users not familiar with Phabricator interface do not easily find the way to comment a task.

The problem origin from the fact that this particular "add a comment" action is not on the menu available on the right, and on the bottom of the page there is a combo box that the users feels the need to select but do not have the "Just comment" item (it has assign/claim, change status, priority, etc. that user do not feel right)