Safari won't scroll through all open windows

Greetings, all.

Sometimes, Safari 5.1.2 won't scroll through all open windows when I use the Apple + ~ command. It will keep cycling through the same two or three windows, ignoring the others (I have to either physcially click on those windows or use the mouse and go through the Window menu). Other times, however, the Apple + ~ command works fine. Any thoughts on why this might be happening?

Thanks in advance for any help.

DPL