Many scanners need a firmware patch to be loaded. I don't have that model, so I can't point you to any exact details of where to find it and where to load it, but a search of the WWW may help (it did for me with a Packard Bell scanner).
If you have a windows software CD for your Astra, I would expect to find the patch in there somewhere, maybe as a .bin file.
You might need to put the patch somewhere *like* (creating directory as necessary)
/usr/share/sane/plustek/patch.bin
and then declare this path in your
/etc/sane.d/plustek.conf
file