Aaah. I had that problem on a canon which I resolved by using command line control:
as root, which worked.
should tell you what devices are available. It's command-line scanning which is perhaps not what you want, and perhaps not an appropriate solution to the problem, but it worked for my purposes. You might need to get into the scanimage manpage if you take this route. With hp, you might need to install some hp specific stuff, but I can't help with that.