Biggest chance to fix it: Change fluid.
Check caliper piston returning (*) them with fingers. It should be hard but same effort on all four.
* when returning brake calliper pistons for replacing pads or anything else, always clean pistons first. Or you will inject dirt into system.
* cut a small chamfrer on new pads. The idea here is ease disc placement when reinstalling wheels. This way you will not need to return calliper pistons every time you deal with wheel maintenance.
