No bootable partition after Ubuntu install

[Doctor Who]

I have installed Ubuntu 8.04 beta from CD onto one hard drive (sda) on
machine with 3.total (including sdb and sdc).  I have other OSs
installed on this machine and use the graphical boot loader GAG to
launch each.  With each OS install, I have the installer put the boot
loader on the root partition of that install rather than the MBR.

This has worked fine until now.  With Ubuntu, it appears that when I
do the advanced options of the installer (step 7 of 7) and I tell it
to put the boot loader on /dev/sda1 it is not being respected as
Ubuntu will not boot with GAG pointing to this partition and the
partition is not being shown as bootable (No asterisk in the 'Boot'
column of 'fdisk -l' output).

My question is:  How do I make that partition bootable at this point.
It's not so much re-installing GRUB as launching GRUB and running
'find /boot/grub/menu.lst' of 'find /boot/grub/stage1' shows that
those cannot be found for /dev/sda1.  They *are* seen for my other OS
installs.

The drive is partitioned as /dev/sda1 as / (10GB), /dev/sda2 as /tmp/
(2GB), /dev/sda3 as swap (2GB), and /dev/sda4 as /home (460 GB).

Thanks for any assistance with this.