Professional development isn't just about the answers. It's also about questions. You can learn a lot from the questions you ask. They can tell you about patterns in your work and in your thinking about your work.
The first trick to keeping a Question Log is to get in the habit of noticing your questions. For many of us, questions zip through our brains without us even realizing them. Make it an intentional practice to notice the questions you are asking yourself. Or try stopping once an hour and reflecting on what questions occurred to you recently. Noticing your questions is a practice you may have to build.