My g/f only eats sweet potatoes and apparantly they make a good roastie but not a good rosti as apparently they have got no starch in them to bind themselves together.
So my suggestions to her was try to to find an alternative binding agent like an egg or something or whatever else ( also told her to put some starch in them from the washing stuff but she just gave me a dirty look)
Anyway hope we have been of some help to you BD