Cassi is showing a tiny bit of interest. Her motivation is not high enough, too much smells on the ground.
I have never had an Aussie puppy that could run on ducks with 15 weeks. Normally the are way to pushy at this age. Cassi still needs some time to develope her motivation and instinct.
Something is happening :-). The fast movement of the sheep is waking up her instincts and rising up her motivation.
Also on ducks instinct is growing now. But it seems like sheep are a higher motivation right now.