I have heard from different leaders that you need to change things. Eat different foods, or try eating your activity points. Sometimes if you do a lot of activity, your body needs to replenish itself so you might need to eat the extra points you have burned.
Another thing is what you stated. Change to flex for a week or two.
Other people say to boost up your exercising more than what you normally do.
Someone else mentioned that she used to walk, and changed her exercise regimen to kickboxing. Maybe do aerobics or something other than walking. I haven't gotten to a plateau yet, but I am getting this advice from the reading I've done on ww online, and what I have heard ww leaders say.
Hope this helps!