Some things to consider:
With bottom coils, if you are taking many shorter, quicker drags, you may get a "dry burn," meaning that you've temporarily depleted the wick of juice, and are getting a bad taste from the burning wick. Longer, smoother or slower drags are usually better with, for example, the ProTanks.
When you are unscrewing your atomizer to refill it, your coil head can become loosened a bit, which will cause problems because it's not connecting properly. Just give it a quick check when refilling your tank, to make sure it hasn't unscrewed a bit.
Sometimes because of particular coil heads, or particular juices, there might be a problem with the wick getting saturated enough, which, again, gives a burning dry-hit taste. With ProTanks, you might remove the little silicon "cap" that sits over the wick part to make it easier for the juice to get to the wick (save it in case you start getting flooding/gurgling and you want to put it back on).
For ProTanks and other atomizers that can use Kanger heads, I've personally had almost uniformly bad experiences with "fake" coil heads, and about 90% good experience with Authentic Kanger coil heads. Try to make sure you are buying the originals. (One "tell" about the fake ones is that they usually don't have the ohms printed on the stem like the real Kanger coil heads do).
Sometimes the pin to pole connection isn't good (the stem of the coil is barely reaching the center pole connection of the battery), so you need to take a tiny knife or flathead screwdriver and very gently jiggle the "pole" or pin connection up on the VV, for example, or take a pair of tweezers and barely, barely pull the pin down on the atomizer. Easy does it! A tiny, tiny bit more connection might do the trick, but you don't want to ruin your battery.
These are just a few ideas. In addition to making sure your battery has a good charge, you should also play around a bit with the voltage to try to get the best vape. Too high or too low can cause problems that might seem they are being caused by the coils. 3.8 is a good beginning point, and then you can dial up or down a bit and see if you get a better vape.