Result slide

When you insert a Graded Results slide, Storyline automatically creates Success and Failure layers, along with the triggers to show the appropriate layer based on the passing score. The Results slide, including those layers, can be customized to look however you want. So one solution would be start over with a new Results slide. 

Otherwise, there are various reasons why your custom failure layer appears. Best guess is that the trigger to show that layer has a condition that is met even when the user passes the quiz. If so, and if that trigger comes after the trigger that shows the custom passing layer, the passing layer wouldn't show long enough to be seen, since the triggers run in order.

However, such troubleshooting is only a guess without seeing the .story file.

Here are the best practices for uploading a .story file:

• Only include slides that are involved in the problem.
• Be sure all objects, layers, motion paths, and variables have meaningful names.
• If there is proprietary content, replace or delete it. For example, replace proprietary text with ipsum lorem text.