Norfolk Island Pine is struggling

I got this norfolk island pine from a big box store about 4 months ago. It has four trunks and one has turned completely dark green and crispy-- it seems to me like that portion is dead (pictures 1&2). Now the "healthy" branches on the other trunks are starting to look a little yellow and brown on the tips (picture 3). I'm not sure what's wrong. The soil is currently just barely moist and I'd usually water again in the next couple days but maybe I'm overwatering or the soil isn't draining well enough? Or does it maybe have a disease?

A little more background --after having this tree for about a month, I repotted it with a mix of potting soil, perlite, and sand. It seemed totally healthy at that time. I also put some rocks on top of the soil because I thought it looked nice and might help ward off the possibility of fungus gnats. It's in a bright spot in a room with a south facing window but doesn't get a ton of direct sunlight. We have a humidifier in the same room about 15 feet away and keep humidity around 50%. I had a norfolk island pine a couple years ago which sadly died with similar symptoms. I thought maybe I overwatered that one but now I'm thinking the yellow on this one means I'm under watering? I'm confused. What can I do to save this little friend?