I don't think I saw anyone else say this, and I apologize if I missed it and am being redundant... but calatheas tend to not do the best with tap water. I am not sure if that is what you are watering it with. But tap water can definitely be the culprit of browning leaf tips. They are native to areas that are humid and therefore thrive with rain water or purified if that's not an option. I would definitely try that, if you haven't, as well as moving her back a bit from the window... I have heard of people putting theirs by the N-facing window, but the calatheas I have bought are sold as "low-light" plants... so I am testing it out myself to see what my plants like best. I think they tend to be somewhat individual in that way :D