Do you know why you are doing usability testing?

The first mistake constantly happen is teams don't understand when usability testing can help and when it cannot. Usability testing is a tool to produce information. However, it can't effectively produce all types of information.
These teams often make the mistake of using usability tests to see how the users "feel" about the design. They want to know if the tested participants will favor the design, want to use it again, and share it with their friends.
While these are all important things to find out, a standard usability test is not the way to do it. There are instances where users were extremely frustrated with the design, couldn't complete a single task, yet told they loved it. There are also tests where the users completed every task quickly and effectively, but hated the design, even though they also told they would use it again. It's very hard to know what to change when you get such results.
Because a usability test allows you to observe the user's actual behavior, its real forte is in telling you where the interface causes frustration. The observation of how users flow through the design provides far more actionable information than asking them if they like it or not.
You can avoid this first mistake by being clear what you want to get out of the test. Posing a behavioral questions, like- "Can our users apply for a car loan without confusion?" or "Will the content simplification reduce calls to our support center?". The more detailed the question, the better the results of the test will be. You will know when the design is working and what to do if it is not.

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.