Foul taste / Joyetech EGO AIO

Hello everyone!

I am very new to vaping and i literally just started and i bought the AIO from Joyetech. My problem is that when i buy a 5 pack coils there is always 1-2 coils that burns on me after 1-5 puffs. Why is this? Is the coil manufacturing bad? I always prime my coils, i never let the juice dry out as i always fill it up before. I keep it clean etc.