Could this have anything to do with being left in the packaging and ignored for 6 years?
I doubt that very much, unless there is evidence of rust inside the brush, it should be as good as the day it was bought.
Without actually seeing it I couldn't say for sure why it's doing all of this, but it does sound more and more like dirt which is very possible if it sat so long.
The best advice now is (if it is possible) While watching Mitch's video and pausing when required, strip the brush down totally and carefully check every piece for dirt, rust or tarnishing, there could be one small piece that's playing up and you will find it with close inspection, check also that the nozzle isn't cracked, as this could also be a factor.
Try to have a magnifier handy, some things can be very difficult to see, once your satisfied that all parts are in order, replace everything as per video after light oiling of course.
Lastly check that the thickness of your paint is suitable for the tip size if indeed there is a difference.
It should work, you just need to be patient and more importantly don't skip any part of the video, and if iv'e missed anything, with a little luck, Mitch or Ace will pipe in because they are both darlings.