Dude the task has criteria on it, if your machine does not meet that criteria then it wouldn't run. Vs running it manually, adjust the settings on the task, like when it runs to a time more likely for your machine to be on. Schedule it to run more often. By default your machine has to be idle for at min 3 mins, for all we know infected with something on your machine sending out spam, so its never idle
Also does not run if on battery, is this a laptop?
As to 10% -- when was the last time it showed that defrag ran? Maybe you just do a lot of stuff on that drive since last week, etc.
The point it the task is designed to run in the background, be it your machine is on when it is scheduled to run or not. It has been this was since vista. 99.999% of the people should have not issues with this and never have to look at defrag.
Maybe your one of those people that like to tweak and have your task schedule service off?