What if our spit never floats? Mine did for the first time ever, but about 20% sank to the bottom. Usually it all sinks immediately, so could this have been a fluke?

That’s great that it floated today. There is lag on the spit test of about 3-4 weeks. So it should continually improve and at least mostly float with just a little bit of sinking. It sounds like you are improving, so check again in 4 weeks.