It's not that the
juice is getting thicker, the gunk on the coil is mostly leftover components that don't vaporize.
This is common with a lot of tobacco flavors, particularly with NET (naturally extracted tobaccos) or WTA (whole tobacco alkaloid) juices, but happens to some extent with pretty much all of them. Some parts of the flavoring components often don't vaporize, so are left behind on your wick and coil. That's likely the "gooey" stuff you're referring to. Coil gunk is something we all have to deal with, but some juices accumulate it more or less than others. If it's getting to be too much effort to keep them clean you can always go out and try some new juices and see if you can find one you like that gunks up less than what you're using now
You already know how to clean your coils, I'm assuming based on your post, so just keep on going the way you are. If you're getting good results then it sounds to me like you're doing it right
