Its been a while since I watched this flick, but if I remember right:
After first using electric guitar mines a few times, then chasing Obi Wan's spaceship blasting a repeating laser gun for 30 seconds from about 10 feet away, Jango finally hits Obi ONCE! Then Boba laughs, and instead of trying to hit him again, Jango says
"Time to finish him off!"
and fires the extremely ineffective heat seaking missiles he'd been saving for some reason.