Sometimes, Photoshop can exhibit unexpected behavior.
One of the solutions is to try to reset the Photoshop Preferences. There are various ways to do it, as outlined here. One way is to hold
Shift keys while the application starts up. You will be asked if you want to delete the Adobe Photoshop Settings File, to which you should answer “Yes” if you are sure you wishlike to have it deleted.
You should also take note, that this means resetting all your settings and preferences to “factory defaults”.
Save For Web
If the problems are only present in the “Save For Web” dialog try holding
Alt while selecting
Save for Web from
File menu. You should be then shown a dialog enabling you to clear the Save For Web preferences. This may not work in all versions of Photoshop, as described here.