Well if you have done everything right with flashtool's installation, then you're doing something wrong while flashing.
If you want to flash a new firmware into your phone you must plug-in your device in "normal" mode (not flash mode), then:
1) Execute flashtool
2) select the "flash device" icon
3) select "flashmode" option, click "OK"
4) select firmware you want to flash (make sure which options you want to be checked/unchecked)
5) click on "flash" button
This is when Flastool will ask you to
->Press Vol- button and connect while pressing (this is flash mode)
Give it a try, also read a little bit about Flashtool procedures.
Sent from my C5302 using xda-developers app