Make sure the printer is installed in CUPS *using the hp:/ backend* (and
not, say, usb:/ or socket:). As weird as it sounds, the sane backend might
not like if it isn't. Make sure the hplip daemons are also working fine
(look for hplip and python hpssd in the output of "ps auxww | grep hp").
There is some help in /usr/share/doc/hplip/README.Debian.gz.
You can use hp-info to check if hplip is talking correctly to your printer.
sane-find-scanner must find your scanner, and xsane (or scanimage, from the
sane package) should be able to scan from it.
If you still can't find a way to make it work, post the output of hp-info
(don't run it as root, run it as the user you're trying to scan as).