This is an issue which has been discussed many times here, particularly since PSP started being produced with a 64 bit version. A search for TWAIN will probably bring up these past discussions. But essentially it is to do with the fact that many scanners on the market these days do not produce a 64 bit version of the TWAIN driver. So if you are using the 64 bit version of PSP it will not be able to use the Scan > TWAIN route as it will not be able to see the 32 bit TWAIN driver. So as you have found, you have to use the File > Import function.
That is certainly what I use unless I open the 32 bit PSP which allows me to use the TWAIN Acquire interface with any of my scanners which only have 32 bit drivers (2 Canon, 1 Brother). I don't mind the Import function since it still brings in the image for editing. But some users complain that the Import GUI is less sophisticated, with less functions, than the TWAIN interface. And I guess it is...
