Unable to edit PropertyField's in my custom editor

I'm writing a custom editor and trying to get a portion of it to show the contents of a class (and have those contents be editable). Unfortunately, while the contents and their respective widgets show up, I can't modify them :(. And thoughts as to why? Here's the code I'm using, 'obj' is a SerializedObject.

 SerializedProperty prop = obj.GetIterator();
 prop.NextVisible(true);        // Grab the first property
 do
 {
     EditorGUILayout.PropertyField(prop);
 } while (prop.NextVisible(false));