Dear friend
I test it on an HP 628 slim USB scanner,Canon 5050 Scsi,Fujitsu 600C Scsi and Canon ??? parallel . all of them work properly with this code . i think your problem is from your hardware . you can test it from other software such as Kodak image ,photoshop , ...
if they can scan your doucument , now you must search problem in your program code.
best regards
reza