That's because shields don't work in Star Wars like they do in Star Trek-they seem to mitigate rather than prevent damage and those TIE cannons do a good job of doing damage (far more than any computer game has it working). When they hit a vital part it does the trick.
But they clearly talk about setting their deflector screens in the battle.
there is logic, listen, thye empire is the controlling power in the universe
right? which means they have next to an infinate number of power rescourses man power, where as the rebellion are flying worn out, almost falling apart fighters through lack of support from other worlds
i'm not saying there all falling apart, or old all i'm saying is there limited by the rescources they have, especially cutting back, maybe only giving shields half the power they were designed for. i maybe wrong but thats what my idea of a rebellion is.