New disc calipers and pads won't go on rotor

[AirSpeed]

Replacing my calipers and can't get them to slide over rotor. I compressed the piston as far as it will go but still won't slide over rotor. Checked calipers and pads and they're identical to the new ones. Notice a thin steel "shim" on the back of each pad, even tried to put pads on without them. I used a c clamp and an old pad to compress the piston, looks bottomed out. Any ideas?