CNX won't use a dust off ref photo if it was created after the images you want to use it with. It has to be created before you start taking the photographs.
If you create a dust off ref photo then use your camera's sensor cleaning feature (assuming it has the feature), CNX will not use the ref photo. You'd have to take another ref photo, refrain from in-camera sensor cleaning once it has been taken, then start shooting the photos with which you want to use the ref photo.
If you have your camera set to clean the sensor automatically each time you turn on your camera, you will go crazy trying to get the dust off feature to work.