Besure you work on right layer
Use Caps Lock to change your cursor
And you can also use preferences and category Cursors to make changes
Image opened from folder.
You use Smart Object (i suppose)
You have to rasterize this object - use Layers panel and use right click to rasteize this layer
you can add new empty layer and check use: all Layers before cloning
OK let's get things straight.
1. jpeg opened from folder into photoshop.
2. No layers, no masks just a normal jpeg.
3. Select clone stamp, alt click on jpeg, black and white cursor arrows show, message saying I have not defined an area.
4. Everything set to default settings.
5. Caps lock not on.
Hooray! Found a solution, not me, Adobe technical.
Did a live session with Adobe and found my right side Alt key was not functioning properly.
Now using my left side Alt key and it works perfectly!