Clean

The IDE attempts to minimize the work it must do to build an application by remembering what has been built previously. If a component does not appear to have changed since it was last built, the IDE does not rebuild it.

Sometimes failures during the build process can fool the IDE into wrongly believing that a component does not need to be rebuilt when in fact it does. You can force the IDE to delete any intermediate files and rebuild everything by clicking Project and selecting Clean... This  also gives you the option of rebuilding immediately.