Clonestamp isnt cloning or stamping !

I unlock the layer, go to the sky to copy the sky across a roofline.
I hold OPT and take a sample.
I move across to the skyline and attempt to apply and the skyline isnt covered. Opacity 100%, Flow 100% Fill 100%. 
Blending mode to NORMAL.

I cannot for the life of me think why this is suddently not working, 20 mins ago on a previous image it was working fine.
Have I accidently hit a settings change?? This is infuriating af.