It could be that the pads have been contaminated and therefore have a reduced braking effect .. remove the pads and throw them in the dishwasher with some detergent and then reface them (once dry) on a flat sheet of 180 grit sandpaper.
Lay the sandpaper down on a flat surface ( I use a bit of MDF board) and with the face of the pads do a figure of eight rotation a few times to remove a tiny amount of the pad face.
Make sure you clean the rotors too, to remove any containments and roughen up the surface of them with sandpaper to remove the polished effect the brake pads have on it.
That should see your brakes working fine again, if failing that, replace the pads