Often callipers with integral handbrake mechanisms wind back fairly easily but stubborn ones result in not being able to get the necessary purchase on both the wind back tool and the calliper to effect any decent movement.

Then Plan-B may be of value

o Remove the calliper carrier
o Remove the disc
o Put carrier back
o Loosely assemble wind back tool into calliper carrier recess
o Refit calliper to carrier

Bingo!

You now have a securely mounted calliper with wind back tool in place and can easily wind the piston back.

It may be that some wind back tools and/or carrier dimensions do not allow this trick but it is well worth the try.