OK, I did search, and came up with the thread "When is "Last Edited" shown" but my question doesn't really follow that topic.
What I'm wondering about is why I can at times edit a posting, and at other times not edit it? According to the referenced thread, people can edit their posting almost immediately, so there doesn't seem to be an edit waiting period. I try to enlarge the screen to see if I'm not moving the mouse to the right place, but the other options always show up, and the "Delete" option seems to cover the little pencil (edit) option.
So what am I doing wrong?