Heat to all the vents.
You can try a quick test - block all of the other vents and turn on the heater - if there is no heat at all to the register in the back bath then something is blocking it. Do not run the heat very long for the test. Don't forget to remove the blocks on the other vents.
In our 360 all the vents work - even the back bathroom. You should feel some heat. There may have been a problem at manufacture. The vent system uses a single duct running from front to back (the furnace is near the front, under the stove/oven). You may want to try removing the register and using a piece of wire (a coat hanger that has been straightened) feel if you are hitting something that is blocking the register. I found have found foam, wood pieces/sawdust, wire, and some strange looking glue when I cleaned out my ducts.
If you do not find anything obstructing the vent - go to the next vent in-line and do the same check. If still nothing, take it to the nearest dealer and ask them to check the run with a scope.