I need help with glow plugs

I recently noticed that my truck is very hard to start even when it is warm. When I start the truck it takes a while then dumps white/grey smoke. After I start it the smoke goes away. I pulled the plugs and tested them they work. Plugged them into the rail while out of the motor and no power. I have power to one side of the GPCM but when I turn the key on there is no power to the other side.