I tried researching this in Adobe Support and haven't yet found an answer that worked, even though there were answers given to my problem on the site. Whenever I use the Clone Stamp or Healing Brush Tool the cursor doesn't change nor does else anything happen when I click on the Alt/Option key in order to define a source point. I am using Photoshop 2020 but I have had this same problem in my Photoshop CS5 before. I have tried everything including restoring everything back to it's original defaults and restarting the Photoshop. NOTHING works...I really would like an answer to this problem because these are key features of Photoshop that I need to use.
If you are using Windows or Mac make sure you are not being effected by a system feature the can be configured the change the cursor size or some system add on that messes the mouse cursor. I have never see a problem in Photoshop like the it when a machine a working correctly. I see some lag about a second with the cursor changeimg to the target cursor in CC versions compared to a version like CS3.