Not all coils are exactly the same when made but you should be getting a bit more life from them. You seem to no your onion's regarding priming etc which only leave Juice as the main culprit. You will find that Sweetners or Sugars in eliquid are one of the main things that cause your coil to start dying on you. PG/VG you are right to use 70/30 juice so that's not the problem. The last thing is Dodgy Coils which can happen on things that are Mass Produced and you will find out that China are not the best for Quality Control, also you can get like anything else Fake Coils but i think they are quite rare.