If this is the answer you get in can mean two things:
a) The item is temporarily out of DS-stock. Probably needs reproducing. Then just put it on the list again with your next order.
b) Some parts are NOT sold individually EVEN IF they have a number in the instructions. When asking you often get the answer that the item is an "essential part of a set" and you don't get them via DS. This is rare, but it happened to some - usually large - items from certain sets.
Of course it could also mean that the item is really "sold out", but as there's a usually a span of 3-6 years after the set has left the shelves when you can still get parts.
As you mention flowers, I would think that the correct answer should be a) and I would put it into the next order again.