Many times while working in Photoshop I grab a tool and it doesn't do what its supposed to do. It can be frustrating and 99% of the time the cause is a setting in the Tool Options that was changed earlier when working with the tool and is still active. It’s a great disruption to the flow of creativity and can just make you angry.
When you encounter this problem the simplest method to get back on track and continue working again is to reset the Tool to its default setting. You could spend a minute or two trying to figure out what's causing the problem but why spend that time searching when you can simply reset the tool with one click?