Sounds like more than one issue.
To show the source area that you want the healing to come from again, try pressing "H" whilst the heal tool is selected. You may have to press it more than once.
If no healing is occurring it sounds like you may have the Opacity on the too,tunred right down. Increase the Opacity slider to 100.....or to whatever setting you find works.
Hit the H key to turn the circles on. Make sure OPACITY is not set to a low number, try 100 first.
Hey Nick, thanks! Yes, that was it
Good, you're welcome.