All of them are full. When you enter the street, they are pulling up into the trees and cannot be brought back down. I haven't tried Andra or Kajuu yet but I'm assuming that they're bugged there too.
Same thing happened to me last time I went snail-gathering. In every street I entered, all the sloths were just pulling back up into their trees, despite no one else being on the street. They then all proceeded to tell me to "sod off" when I knocked. Every freaking one.
This is a bug. I just got 6 streets of "full" sloths in a row and there aren't other players around. The moment I walk up to the tree, the sloth vanishes, with a quick, odd-looking animation. When I knock I get the "full", "sod off" messages.