Spot removal tool stopped working abruptly few weeks ago. It doesnt do anything in either Heal or Clone mode. My feather and opacity are at 100 and size around 50.
Any ideas how to troubleshoot / fix this issue ?
Try setting the feather to a lower value and make sure that the size is bug enough to see it. At feather 100 the clone or heal has little effect as it blurs the transition very much and you only get any real effect in the middle of the circle which might not look like much. Most of my clone healing of dust spots is more around a feather of 40 or so.