Depending on the sugar content of the fruit, it might not sink right away. Don't worry about it, "have a home brew."
The CO2 should keep the top part of the fruit preserved until it can get water logged enough to settle. Plus, since you added it to the secondary, there should be some alcohol, which can kill germs and further preserve the integrity of your beer.
Just wait it out. Don't worry...it's homebrewing, not rocket science.
May the Yeast be With You!