I have Photoshop Elements 12, using it on a PC. The tool worked properly until yesterday. Now it only allows me to make an initial selection. Using Alt will allow me to deselect areas, but once I made my initial selection, the quick selection tool no longer will allow me to add areas.
If your program is Photoshop Elements 12, you have posted in the wrong forum. Some how your thread got posted in the Adobe Premiere Elements Forum. Please post your thread in the Adobe Photoshop Elements Forum.
Putting on my Photoshop Elements hat for a moment
a. have you tried Reset Tool
b. have you deleted the Editor's settings file...Ctrl+Shift+Alt (for Windows)
c. have the Add Subtract buttons in the Options bar stopped working....what happens if instead you hold down the Shift key of the computer keyboard when you want to add?
The Adobe Photoshop Elements Forum team should be able to sort out your your Photoshop Elements issue.