Why usability testing many times does not yield good outputs?

Usability testing is a tool to produce information, from which the designs are made or improved. Hence, the key lies in how one conducts the usability testing. The following are important parameters that one needs to consider while doing usability testing-

1. Do you know why you are doing usability testing?
2. Bringing the usability team together
3. Get the right participants
4. Design the right tasks
5. Facilitate the test effectively
6. Plan on how you will disseminate the results
7. Iterate to test potential solutions

What is a successful usability test?

When a design has a usability problem, it is because someone made a wrong decision. They chose to take the design in a direction that creates frustration for the user. A different design choice would have prevented the frustration.
There are two outcomes from poor decisions: either the user experience is worsened because of a change that just shouldn't have happened; or a valuable opportunity is missed to improve the design's user experience. Either way, when usability tests work, these results are significantly less likely.
I consider a usability test to be successful when the design team members receive the information they need to make the right decision. Successful usability tests produce informed decisions.

What is good design ?

The flow of creating something seems to follow the sequence- a thought to idea to tangibles. Tangibles can be something of value that one can feel or sense. From an idea to tangibles means various intermediate steps. The clarity on how to come up with these steps and laying it out on a media to see the manifestation of the idea is design. So, what is a good design? A good design is one in which the degree of closeness between the idea and its manifestation (tangible) is as close as possible.

On the other hand if the closeness between design and its manifestation is lacking then no amount of analysis and parameter tuning can markedly improve the value of the tangible. Which also means that there are fundamental contradictions designed into the manifestation. The ability to preemptively address and resolves design issues is an area of great importance.