That should certainly work.
I just tested with my library and added the keyword 'Wout van Aert'.
The I could find it easily in the Keywords HUD:
The keywords are stored in your Aperture library in internal propertylist files. If the HUD cannot search the keywords properly, these library items may be incorrect.
Make a test - create a new Aperture library and add a few images. Define a few keywords. Does this work in a new library?
If yes, then use the Library First Aid Tools to repair your Aperture library, or to rebuild, if necessary.
see Repairing and Rebuilding Your Aperture Library.
To repair or rebuild your Aperture library
- Close Aperture, if it’s open.
- Locate the Aperture library you want to fix, then hold down the Command and Option keys while double-clicking the Aperture library.
- The Aperture Library First Aid dialog appears.
- To repair the permissions of the files within your Aperture library: Select Repairing Permissions.
- To rebuild your database from scratch: Click Rebuild Database.
Before rebuilding, make sure you have a current backup.