I have a dwarf cavendish that's about 3.5-4 feet tall that appears to be sick, but I can't tell what from.

Positive signs:

- New leaf growth is coming in and is growing the right shade green. Four or so new leaves are growing right now.

- Pups are popping in (two earlier in the summer, one now) and growing fine and continuing to grow fine when moved into their own pops. The newest pup has a few light brown spots on the middle of one of its leaves, the previous two have been perfectly healthy and hardy.

Negative signs:

- Mom seems sick. Most of the old leaves are experiencing browning around the edges. Some have just crumpled up and dropped entirely. I have been pruning the leaves when they get significantly brown. Most of the leaves that are dying were present when I purchased Mom last fall. Because of the leaf death, the pseudostem has become less thick than it used to be, but still feels thick and firm in general. There are only maybe 3-4 mature leaves left.

- New leaves, when they come in, are missing maybe their bottom half or bottom third. The portions of the leaf furthest from the pseudostem come in fine and fleshed out, but the portion of the leaf closer to the pseudostem is totally missing. One leaf that came it was basically a sloppy, muddy brown at the bottom as it was growing.

- If I feel down inside the pseudostem from where the new leaves emerge, the tree seems to be covered in brownish-black, very moist/wet slime. It's like the center of the tree is a muddy compost-y slop. I think this is why the new leaves have been half missing; they end up kinda turning into wet mulchy mush inside the tree.

The newest set of 3-4 leaves that are growing are a bit too dense to really reach into so I can't see if this problem was persisting.

I live in a city with a moderate climate; it has been 10-20 celsius pretty much every day this summer. In the winter, it is maybe -10 celsius. The banana tree has lived indoors the entire time, our house is never colder than 15 in winter.

The tree is in a south facing window, albeit one that doesn't get perfectly unobscured light. It was originally in a north facing window, but I moved it around six months ago.

I have used a balanced fertilizer, dilluted, maybe twice this summer but not at all recently. It is watered once a week or so, only when actually dried out although it's possible it has been overwatered.

Any idea what I could be doing wrong?