sy/sx (-1) in UV window sometimes not working

Well, I know the title is classy. However, I often run into the problem that both these very useful hotkeys stop working in uv window for no apparent reason from time to time. In general they work , so I guess i might hit another hotkey that more or less deactivates those. However, I have paid attention to it and still couldnt identify the cause. Restarting Blender works most of the time. I cannot recall having this problem in older builds ( at least pre 2.64 ) so MAYBE someone else has an idea what´s going on with those hotkys ?