First question that I have for you is did you look in the help files?. Simply type clone stamp in the search After Effects Help field at the top right corner of the application frame it you will find all kinds of information. There are a few good tutorials linked directly from the help file. After Effects Help | Paint tools: Brush, Clone Stamp, and Eraser
Because After Effects works with video you have to make sure you have the correct options for your clone stamp or your paint tool selected and you are working in the correct window. It is not so much that the clone stamp is not working, it is just that you probably do not have it set up correctly.
Yes I did look at help files and tutorials. I've been using After Effects since version 6.5 so I know my way around it and have no problem with "set up correctly" that you mentioned. But I just never have to use the clonestamp tool until now.
I can use the stamp tool, and I can see the source gets applied to target as I am "painting', but as soon as I let go the mouse, all changes disappears on current frame, back to original . On timeline I find keyframes are created. And if I scrub along the timeline I can see the changes applied to those subsequent frames. I don't think this is the expected behavior, as it is not what I saw how it worked on those help files/tutorials.