Grapefruit tree advice

I’m looking for advice on what’s wrong with my grapefruit tree. I have a lemon, mandarin, and lime tree in other pots in the same space that have been growing well, but this tree eludes me. Her leaves have been falling off but her stem seems strong. She is in a 24” pot, gets watered when the soil gets dry, and gets sunlight all day (given whatever weather conditions persist and what sun makes it into my apartment). I bought her from “fast growing trees” about 8 months ago at the 3’-4’ in height, and she is still 3’