Try this (assuming that /var/cache/man/whatis does not exist yet):

# man man
[ fine if a bit slow ]
# man -k man
man: nothing appropriate
[ **after** /etc/cron.daily/makewhatis.cron run from cron 
 earnining you only a bunch of errors in mail ]
# apropos man
man: nothing appropriate         <--------- this is the problem
# sh -x /etc/cron.daily/makewhatis.cron
# apropos man
[ now it works with a vengeance ]