We all have our days. Sometimes it will be easier to focus than others. Don't get discouraged. Instead, take action. Here are some things you can try to address your issues with focus:
Change the questions you are using
If you are using questions to change your focus, but they aren't working, then try some new questions. This is the simplest way to really hone in on which questions help you achieve the best focus, and the best part is they should be highly reusable, every time you need them. You will get better over time.
Change the time frame
If 20 minutes is too long, then try 5 minutes. If 30 minutes is too short, then try 40. Experiment to find your sweet spot. For example, for many people, their sweet spot is 20 minutes of intense focus, and then take a 5 or 10 minute break.
Change your approach
If a time limit isn't working, then try a quantity limit, such as "identify five items." If a quantity limit isn't working, then try a quality limit, and see how good you can do. Find a way to connect it to your values. For example, if you like adventure, than make it an adventure. If you like to learn, then make it a learning exercise. If you like excellence, then make it about excellence.
Change your pace
Try speeding up or slowing down. If you can create a sense of urgency, then you may find it easier to stay fully engaged. If you find that creating a sense of urgency stresses you out, then slow down you r pace and focus on skill and quality.
Change the time or change the place
When all else fails, one of the best ways to change your results is to change the time or change the place. Maybe you can focus better at night or vice-versa. Maybe you need to try another room, or try another place entirely. The key is to find what works for you, and then to understand why it works for you.
One thing to keep in mind is that stress can work against your focus. If you keep finding yourself in stressful scenarios, than remind yourself to focus on what you control and let the rest go. One of the best ways to deal with stress is to take action, and if you take focused action, using the steps above, then you can help cope with any stressful situations that you face.