Why can I draft a building but not design one that works?

You have confused two different skills. Drafting is technique — you can be told what a room needs and produce it correctly. Designing a building that works is judgment — knowing what a room needs before anyone tells you, because you have imagined the tired, the hurried, the confused person who will actually stand in it. These are not the same muscle, and no amount of practicing the first builds the second. You will be tempted to fix this with more software, more precedent images, more hours alone at the desk. None of it will work, because the gap is not technical. It is that you have been testing your designs against the drawing, not against use. What you actually lack is contact — with the bodies who will use the building, and with colleagues who will argue with your assumptions before the concrete sets. This is not solved alone. Bring others into the plan while it can still change.