403 forbidden access with apache2

Hi. We have a 200 user LAN under NIS and NFS with apache2 under 9.1 with all the YOU updates. The apache2 is untouched from when installed. Some users can access their files in their public_home directory via the server and others can't getting a 403 message from Apache2. The permissions on the respective directories are identical. Can anyone give me a starting point to tackle this one? Why can some gain access and some can't? Cheers, Steve.