First check your disc hasn't got a bend in it. If it has use an adjustable spanner to grip the disc and bend it back. It will take a few goes and you'll not get it micro perfect but it will be enough so don't over do it.
Pull the pads out and soak a cloth in white spirit and drag it through to clean up the pistons of brake pad dust. This often gathers round the edges of the piston and causes them to stick.
While the pads are out, GENTLY & SLOWLY pump the brake lever so that you can see which piston is sticking. Then use a knife or thick alum key to push and hold the good piston back into its housing. Whilst holding the good piston in with the alum key,GENTLY & SLOWLY pump the lever again so the sticking piston comes out of the housing a bit. When you have revealed SOME of the outside edge of the sticking piston use a silicon lube and an ear bud to lube up the edge of the piston. Push both pistons back in fully to their housing and then keep repeat the process for the good piston holding in the sticking piston. Keep doing this for both side until you start to see equal movement in them both when you pump the lever (SLOWLY). Remember to push the pistons all the way back in before you put the wheel back on but you may need to bring them out a couple of millimeters so they don't stick in their housing off the first ride.
DO NOT OVER PUMP THE BRAKE LEVER WHEN YOU HAVEN'T GOT THE WHEEL IN, OTHERWISE THE PISTONS WILL FALL OUT AND YOU'LL GET AIR IN THE SYSTEM AND YOU'LL NEED TO BLEED THE BRAKES.
Apologies if this is all grandma egg sucking... all works for me and my F1's.