Rollout XP SP2 using SUS

I am currently rolling out XP SP2 using SUS. Most
clients so far (30+) have worked. However, I have a few
that will not work. These clients have been patched by
SUS before, with other Microsoft patches, however, they
do not work with SP2.
The error I get in the event logs on the clients is:
Event ID 20
Installation Failure: Windows failed to install the
following update.
- Windows XP Service Pack 2.

That's it. No other messages.

Any help would be appreciated.

AJD