Mine took a lot of fiddling, patience, swearing, playing around with different washers...
Push the pistons all the way in, use a flat head screwdriver and push one pad onto the other to push the piston in. Then put in the selected washers to get the caliper sat so the rotor is central. Then pump the lever, see if the pads hit it square, and if it rubs.
I had to keep doing that, until I got the combination of washers. Even took a file to the mounts on my frame as I am yet to get it faced! Had to do this for the front and rear disc too!
Just take your time with it. Also make sure the rotor and pads aren't contaminated.