What is your iPhoto version? Are you usng the latest iPhoto 9.4.3?
When you import into iPhoto, the new imported photos will usually appear in a new event. Even dragging a photo directly onto another event will not import into that event but rather create a new event. To add the photos to an existing event, merge the new event and the old event by dragging the new imported event onto the old event in events view.
You should be able to find the new event by selecting the imported photo in the "last Import" album and to use "Show Event" from the pop-up menu.