apache 2 expiresdefault

hi,
i have a linux/apache2 newly setup. i put it online last week but after a day, i got a lot of email/phone complaints saying they are seeing old pages/old contents of the site. My site changes frequently(daily). after checking i found that apache has ExpiresDefault A2419200 which spells to 28days. I changed it to ExpiresDefault A0 and reload Apache. Some end-users are now seeing the new content but some are still seeing the old pages. What else i missed?

thanks,
jake