I thought you had to left click on the area that you want to clone or heal.
no… enter usually works too and has since I bought the program in December, but now nothing works to make the selected area be replaced with the new area.
Check the opacity slider.
Yes- [Enter] works for me (CC2015.8 Win-10)
You do need to L-Click to set the area you want to 'disappear' (call it the destination), you can then move the second circle that is the 'source' of the clone.
Also Press '/' repeatedly to make Lr auto-select different places for the 'source' circle.
[Enter] actually 'closes' the selection and starts a 'new' brush to clone another area.