When you change pads you have to bed the new ones; pads that haven't been bed correctly will never achieve full power. You can save your current ones by removing them, scuff the surface with some emery clothe and then do 10-20 hard stops from about +15mph on each wheel without locking the brakes.

Other than bleeding 2-3 times a year, always bed new pads and keep current pads and rotors free of soaps and lubricants. Have a look at your hoses every once in a while too to make sure there are no leaks or bubbles.