One possibility is that names are colliding — we rely on Sketch's built-in exporter, which doesn't give a lot of flexibility with names.
E.g. if you have two elements named the same, you'll only see one of them inside Haiku.
If your slices don't have unique names, (e.g. if two of them are called "Oval") then Sketch's built-in exporter will cause one to overwrite the other.