I wonder if the feed you are listening to only streams a single channel. Here in my area (Sacramento) all radio traffic is placed on a single channel overnight. This is done because the traffic is light and therefore only a single dispatcher is needed.
I can imagine that in Tacoma they'd do the same thing. During the day traffic is separated onto many channels, but only on one at night. And if the stream only has the "other channel", you'd hear nothing.