Ironically there is little academic literature on policy success!

It is easier for everyone to focus on the failures.

A policy is successful insofar as it achieves the goals that the proponents set out to achieve.

However, only those supportive of the original goals are liable to perceive, with satisfaction, an outcome of a policy success. Opponents are likely to perceive failure, regardless of outcomes because they did not support the original goals.

Politics is cruel. Opposition will rarely want to give any credit. The media is generally negative. Even if generally it is accepted something was successful, there will always be those who said it should have been delivered quicker, more cheaply, and in a different, non-specified way!

Furthermore, increasingly I would argue that too much policy is driven by Think Tanks, lobbyists and a political agenda that bears little connection with those it is meant to serve or deliver.