During the test, you'll be asking the participant questions according to the script you defined previously. As participants encounter problems, don't make a literal interpretation of what users do or say. You need to look for the underlying problems that caused the issue. If a user didn't tap on a button, you need to figure out whether that was because the user didn't pay attention to the button, the user recognized the button but the user thought it was for something else, or the user didn't expect the functionality to be provided in such a way.
Some users may even suggest a specific solution to you. For example, the user having problems in finding a button may suggest to make it bigger. However, that may or may not be the best possible solution. Use the suggestion to understand what is the root cause and what is the user trying to achieve. Ask in which ways the proposed alternative would help the user to achieve their goals and try to rephrase the suggestion to connect it to a more general problem--is the request intended to make the button easier to access since it supports a functionality that is often used.
As users experience issues during the testing session, you may be tempted to provide them some help. Remember that the goal is not for the user to succeed, but rather for you to learn what fails and why. Even if users get blocked for a while, use that to understand why they are blocked and let them find alternative paths. Users may ask you for confirmation--should I tap there?--or help--where can I find the map?. In those cases, you can turn the question to them--what do you think? where do you expect to find the map?
If users get really off-track, you can move on to the next task. Make sure that you remain positive and not make them feel like they failed or are doing something wrong.
As participants are answering your questions or describing their thoughts, try not to interrupt them. Give participant enough time to express themselves. You can also use any silences to your advantage. After a participant answers your question, you need to keep quiet, as at that point, some more details about the previous idea may come to the participants mind, including details, nuances, exceptions, and other useful information for you to learn.
If the participant digresses, starts providing feedback based on their opinions, or guesses how the product would work for other people, make sure that you direct the session to focus on the participant behavior and experience, not their opinions. Ask them to show how the aspect that they describe would work for them in the current scenario.