I think it helps if you make the dry sugar wet, at least until the bees get
the idea that this lumpy stuff is food and not debris to be removed.  I cut a
large X in the side of a kilo bag of sugar, fold the corners back, and gently
pour in water until common sense says "stop".  The bag is then inverted over
the feed hole in the crown board.
If at a later visit I squeeze the bag and find an area of hardened sugar it is
a simple matter to make another X and pour in water from the top without
disturbing the bees.
Chris Slade