The CW80 is perfectly capable of powering the milk car and more at the same time. If the CW80 and operating track and log car all work just fine, and then you change one thing (log car to milk car) and things stop working, I would look at what changed (milk car) before blaming what hasn't changed (transformer & operating track).
As such, it look like your milk car needs to be investigated. Does it work on another layout/setup? Does another milk car work on your setup? Does an inspection of the milk car wiring show any concerns and is it correct?