Do moss balls float?

I've had three moss balls for about a year. I don't "keep" them in a proper aquarium with fish or anything, just in a large glass with rocks and water. I know it's normal for them to sometimes float to the top after being rinsed/rolled, and then they need to rehydrate, but it's been getting worse. I haven't changed how I clean them—let's say, changing the water regularly, etc. Nevertheless, all three are always floating on top now. At first, it was just one, the smallest. The two larger ones sank again after several days, and the smallest one is still floating on the surface. At first, it only took a few hours, and as I said, only one. Why is that? Am I doing something wrong, or does this indicate that they're unwell?