Hi Arthur. You’re not really giving us enough information about your problem, therefore I’ll have a guess at what’s happening here.
Are you by any chance applying Sharpening or DeNoise, (for this example I’ll use Sharpening) to the image when this happens.
If so then your problem could be you have the Mask setup for Select Subject because.
If you have a very busy image or the person you want to sharpen is small in the frame, then sometimes Select Subject may get confused and only pick a small part of that image or miss the person completely and choose something else, that you don’t notice.
Meaning when you’re viewing the before and after side by side you may believe nothing has changed.
To fix this simply select Sharpen to open the Sharpen sidebar, then pick Selection Type and choose ALL from the drop-down menu and try again.
I do hope this helped rectify the problem you were having