How "hard" is your water in Wisconsin?
The flow is usually (not always, when the piping doesn't support it) in at the top, one end and out at the bottom, the other. If the upstream half is cold, the bottom is probably clogged near the input. If the downstream half is cold then the (large) bubble at the top is probably being pushed towards that end and the water never gets to the top of those tubes. When the rad water level is nearly down to the tops of the tubes, it ends up as a series of weirs with the water level getting lower and eventually stopping water getting over the tops of some downstream tubes.What about one side not heating.