We have 3 of the 3500 model switches in a stack. The 2 lower units come on immedialtely when powered with their each 110 volt power plug. The top 3500 switch only also has the 12 volt add-on electric power plug on it, this top switch has 110 volt/220 volt auto sensing power plugging. When the top switch is plugged in it is taking 10 minutes for it to power up. Should it take this long to power up? Is there any chance that the top switch isn't powering up and getting electric power from the POE from the 2 lower switches?