Do you clean the pads and rotors as part of normal maintenance?
If not the pads get covered in dust, etc. which reduces performance.
I spray a water/alcohol mix on a clean cloth, folded in half, and draw the damp, folded cloth back and forth between the pads. I repeat until the cloth is clean after running between the pads. I do the same to the rotors.
I do this every couple of weeks or so when performing normal maintenance...cleaning and lubing chain, etc.