Spot removal: Not working - has it changed?

Hi all. I've noticed a change in the spot removal tool. For example with a black spot on a pure white background, if I try and clone in an area of white, the black spot simply gets lighter but it remains. Previously, if I did this, the spot would be completely erased with whatever colour I chose. I've tried minimum and maximum opacity but this doesn't solve the issue - the spots are still there. Any idea's what's causing this? Pretty sure something changed in the last update or two as it's been working fine for years!