I think rather than how thin or thick they are what really does it is sweetness and darkness of the
juice. Thinner juices should vape away better than thicker ones but the brown stuff comes from the color of the
juice and or even a clear juice that doesn't vape clean and gets burned on. You have wicks in there and they don't burn, they get juice stuck to them and it's that which burns and creates gunk.
You can clean your atty's ya know, that may help.. there are several threads with cleaning methods. If it is something about her juice it must be something special she is mixing with the juice but most likely it's the color and or sweetener used that starts to build up and cause problems.