The/a pad/s are/is more than likely locking in the caliper body, they do this by twisting and/or rocking on the pins.
Some pad manufactures dont get the dimensions quite right i.e. not long, thick, wide enough. Which allows slightly more movement within the caliper body over time they actually damaged the working surfaces, tell tail signs are you can see the coating has been removed at each end and/or groves being worn into the caliper body.
These groves allow a pad/s to twist and lock causing binding. The piston/s still do what there meant to and retract this leaves a clearance between the pad/piston next time you pull the level it will feel like there's nothing there and a real lack of power, caused by the piston/s trying bend the pad!
If this is allowed to continue and it only takes a short while, the pad that's locked wears at whatever angle it's locked into which further reduces the power that can be applied, plus the discs can over heat and wrap which magnifies all the above.
The angles are small as are the clearances often they can't be seen (if you don't know what your locking for) and all appears as normal check the pad/s if it not worn square/parallel they're locking. Locking can also be caused by just really dirty calipers and salt and year of poor maintenance.