Problem: Some polygons and lines do not display labels in the map document
After turning on the labels for either polygon or line feature classes and specifying that there should be one label per feature part, the labels do not display.
This can be caused by geometry errors in the polygon or line feature class.
Solution or Workaround
In ArcToolBox navigate to Data Management Tools > Features > and run the Check Geometry tool.
If geometry errors are found, use repair geometry tool located in the same toolbox to fix the error.