A USB-connected device? Anyway, some ideas to start with...
1) Try removing ~/.sane (ie any bad user config that may be causing issues), then start your scanning application. Any different?
2) Start simple-scan from a terminal in debug mode
Observe/capture what is reported.
3) Are you using HPLIP? If so, you'll be using the CUPS 'hp' backend to print. The 'hpaio' scanner backend relies on a correctly configured CUPS URI (ie with the hp backend) to facilitate communication. So, in this instance check the URI defined via
The device should be enumerated (when attached and turned on) by running the hp backend manually like this
Code:
/usr/lib/cups/backend/hp
Also, for HPLIP the 'hp-doctor' diagnostic can be helpful in determining what might be wrong.