I’ve tried opening your project, and the completion seems to be working just fine for me.
The only thing I can think of is to try to delete your entire NB cache, which should be located in $HOME/.netbeans/$VERSION/var/cache/
. This is a wild guess
Create a backup first, I didn’t try this!
If that fails, maybe you should try creating a new project, maybe that will kick NB in the butt.
Note that in NB 7.2 beta, the cache has moved to $HOME/.cache/netbeans/$VERSION.