You would want to check that the objects you are attempting to process are indeed paths. This can be easily checked by using the node tool (F2) and clicking on each object in turn. When clicked, you should see the nodes for the paths.
If the objects are not paths, you will not see nodes. Return to the select tool (F1) and click each item, then use Path, Object to Path to convert them. You can select more than one object by holding Shift while selecting, and convert them all to paths.
To further ensure confidence in your ability to perform the desired difference action, create simple objects (squares, circles, combinations), convert them to paths as noted above, then perform the action again. This should result in the "textbook" shapes and assist your learning experience.