If your shower knob isn't turning all the way, there are a few things that could be causing the problem. The first thing you should check is the water pressure. If the water pressure is too low, it can prevent the knob from turning all the way. Another possibility is that the washer is worn out or the valve seat is damaged. This can usually be fixed by replacing the washer or valve seat. If neither of these solutions fix the problem, then it's likely that the valve itself is damaged and will need to be replaced.