First, check the "polarity" of the wires to the track. I presume you did check this as you stated switching wires.
I'd also suggest that you swap sides as well... so if using A-U, go try D-U (this is because the ZW actually has 2 rectifiers, one for each channel. It wouldn't be unheard of to have one go bad)
Additionally, the ZW whistle control has two positions... Most modern whistles only need the whistle switch to be activated part way.
