That is because Jake blocked the hole as Finn jumped down there, depriving her of oxegen. So the flame started to go out because it was not hot enough. =I If it wasn't for Jake she would have melted to the Earths Core.
But it seemed like she stopped falling a decent amount of time before Jake's clog and Finn catching up to her.
Even though Jake cut off the oxygen, I figured it'd be a gradual process of her burning the remaining oxygen until she isn't hot enough to sink anymore. Not Finn just finding her dim flame shortly after the clog.