It's hard to say. If it was thin then it would wick easier so it shouldn't cause burning so easily. I use very very thick liquid and it can gunk up the coil very quickly. It depends on how "sugary" the
juice is, how dark it is etc as well.
Have you taken a fluxomier's seals apart before? You just pop them off with top with something that can get inside. The last one in there, the one with two slots either side that the wicks come out of? You can cut those slots so they're /\ shape, just make them a tad wider and it'll wick any thickness of liquid if you tilt it upside down in seconds.
It takes a little getting used to having a super wicking fluxomiser but it's a good thing in my book
If the coil is black or dark brown, it's pretty normal for it to happen. If it's starting to taste bad then yeah, dry burn it as clean as you can and it should be better. You can keep doing it until it's spent completely. I get about 2 - 3 months out of one fluxomiser but I dry burn them to death lol!
ps, on your very first fill, make sure you put in a drop or two right onto the coil before you use it so that it does not burn from first use. It will last longer and wick better from the start that way.