Heal and Clone Brush

Just started to use PL again and jumped to V3. Are there known issues with the clone and heal brush? I have fo found that when cloning the source area sometimes behaves in the same way as the clone (the program selects an area) and when using clone it is erratic in picking areas that match and sometimes what goes in as the replacement is not the same as what is automatically or manually selected.

Solved. A back-up program was accessing the external hd where my images are stored. That compromised performance big time. One tthing though, when using the heal/clone tool a black bar shows down the side of the image. It goes away when I close the tool and does not appear to impact the editing.