Is this script in a non-SUID filesystem perhaps? Have a look at the mount options for the filesystem that the script is in. You can probably get it by:-
df /path/to/your/script
You can then review /etc/fstab, /etc/vfstab or /etc/filesystems as applicable to your operating system (you don't say which, so these are the most common)
You might also get the same information from the mount command by itself, but this may be OS specific, so I'd be less sure about that.