What power source do you have it plugged into? If you don't have the Direct-To-The-Battery kit that Brake Buddy offers, you could be seeing a power shortage. Most newer vehicle "Power Outlets" are 10-amp with skinny wire.
BB has a price on the kit, but if you call mentioning trouble, they'll send you the kit free.
Counter-intuitive, but they told me the logic of brake application uses more power than the compressor. Go figure, but that's what their tech said.