Clone tool not working - help very much appreciated

Hi folks,

For years I have used the clone tool happily which has been great for touching up photo's on Photo-paint 12

A few days ago - imported a jpg into Paint as usual, went to use the clone tool and it just doesn't work any more. I can select the icon and the little circles come up but it just doesn't 'clone' anything. Its as if there is mysterious setting that has been turned on that just makes nothing happen.

I have reinstalled the software: no help - still the same.

Almost screaming at the computer screen now in frustration 

Any ideas anyone??!! many thanks in advance.