On my Voltage, when the main salon A/C comes on there is a delay (40 seconds or so) and then you hear the fan first followed a few seconds later by a louder hum (the compressor kicking on). We have the Domestic 15k ducted unit. Make sure that you hear the hum of the compressor, and that will tell you if the roof unit is trying to work. I have heard that these are sealed units, and very few people actually recharge them, but most have to replace the whole roof unit. Here in Oklahoma, when the air is on, it drips water. If your front unit drips, but your main unit doesn't, there may be something worth investigating. Don't panic, as it could be something simple like a dirty condenser, or a loose fan not blowing like it should.
Hope this helps to troubleshoot your problem.
2013 Voltage 3905