Icing conditions can also be a factor; freezing rain in the high 20’s F (~-4C for those who can’t F) will build up a layer of ice on the outside unit especially since it’s already the coldest thing around. That ice prevents air from circulating through, so it loses effectiveness. Running the strips for about a half hour or so, then running the air conditioner for about a half hour will defrost it quite effectively and restore it to efficient operation. It is my understanding some systems even do this automatically.