I take the pads out, then press the pedal softly a couple of times. It's very easy to see which piston isn't reacting.
Then I put three pads back in, and a space in the fourth to stop the piston popping out, then get the piston well out, and give it a thorough clean. Maybe a lick of brake grease.
Then after doing all four, I check again, and make sure the pistons emerge at the same rate. Then as noted above, make sure the pads don't stick in the calipers, and usually that does it.
