We can base our assumptions on the wrong knowledge. Our general knowledge of the good could be faulty therefore if we start out with something that is wrong, we can end up with the wrong choice.
Our knowledge of the particular circumstances could be wrong or our reasoning is faulty but we might still achieve the outcome desired.
We fail to deliberate at the right time or take too long.

Practical wisdom means we have the right intention, identify the right means and doing so at the right time. Practical wisdom deliberates from the "good life"

“Feeling lost, crazy and desperate belongs to a good life as much as optimism, certainty and reason.”

― Alain de Botton