Which version of flashtool are you using?
Sent from my C6602 using xda premium
I'm using flashtool 0.9.10.1 on Windows 8 64-bit.
EDIT: Here's a log that may help
19/002/2013 04:02:35 - INFO - <- This level is successfully initialized
19/002/2013 04:02:35 - INFO - Flashtool Version 0.9.10.1 built on 2013-01-09 19:49:47
19/002/2013 04:02:35 - INFO - You can drag and drop ftf files here to start flashing them
19/002/2013 04:02:40 - INFO - Device disconnected
19/003/2013 04:03:05 - INFO - Selected C6603_Generic Uk_No Branding.ftf
19/003/2013 04:03:05 - INFO - Preparing files for flashing
19/005/2013 04:05:46 - INFO - Please connect your device into flashmode.
19/006/2013 04:06:02 - INFO - Device connected in flash mode
19/006/2013 04:06:02 - INFO - Opening device for R/W
19/006/2013 04:06:02 - INFO - Reading device information
19/006/2013 04:06:02 - INFO - Phone ready for flashmode operations.
19/006/2013 04:06:02 - INFO - Current device : C6603 - CB5A1PKLNS - 1270-6350_R8A - 1269-5309_10.1.A.1.350 - GLOBAL-LTE_10.1.A.1.350
19/006/2013 04:06:02 - INFO - Start Flashing
19/006/2013 04:06:02 - INFO - Processing loader
19/006/2013 04:06:02 - INFO - Checking header
19/006/2013 04:06:02 - INFO - Flashing data
(It doesn't get pass this step and stays at 0%)