whole disk encryption -- not prompting for passphrase

[Hatta <hatta00@fastmail.fm>]

A short while ago I set up a system with whole disk encryption.  I used 
the guided LUKS
partitioning sytem to make 3 encrypted lvm volumes, root, home, and swap_1.

Anyway, I broke that installation, so I needed to reinstall.  I used a 
netboot image in expert
mode, manually opened the encrypted volume, and installed a new system 
to root. It seemed
to go fine, but on rebooting it does not prompt me for my passphrase, 
and it cannot find the
root filesystem to mount, I eventually end up in busybox.

I can mount my encrypted volumes just fine from a live CD, so they're 
ok.  So there's something wrong
with the boot process, debian's just not finding it.   Here's the error 
message I get:

Begin: Mounting root file system ... Begin: Running /scripts/local-top 
... [5.769352] device-mapper: uevent: version 1.0.3
[   5.771151] device mapper: ioctl: 4.13.0-ioctl (2007-10-18) 
initialised: dm-devel@redhat.com
   Volume group "debian" not found
   Volume group "debian" not found
done.
Begin: Waiting for root file system ... done.

And then I just wait for BusyBox.  Anyone have any idea what's going on 
here?