If I understand it correctly - sometimes when you have a fast moving bullet it can jump through its target and not register it as a hit.
Say your bullet is 5 pixels long but is travelling at 20 pixels a frame and your target happens to be in that gap of 15 pixels ? No hit !
Just been playing with this problem and came up with the idea of firing either
a second longer (and invisible) bullet which would give you a better chance of catching the target
or
altering your bullet to make it have a long empty space trailing behind it but this empty space would still be covered by the collision polygon.
anyway a quick example of the invisible bullet idea.
Please see attached capx