F250 Diesel

I have a 91 F250 diesel with 439,000 miles love the truck, but when it gets cold out it is hard to start, like it is not getting fuel, glow plugs are new, and the engine is warm. What is the problem? any help would be greatly appreciated. Thanks

I have a 91 F250 diesel with 439,000 miles love the truck, but when it gets cold out it is hard to start, like it is not getting fuel, glow plugs are new, and the engine is warm. What is the problem? any help would be greatly appreciated. Thanks
DOES YOUR FUEL HAVE ADDITIVE IN IT AND DID YOU CHANGE ALL OF THE GLOW PLUGS AT THE SAME TIME?

DOES YOUR FUEL HAVE ADDITIVE IN IT AND DID YOU CHANGE ALL OF THE GLOW PLUGS AT THE SAME TIME?
Yes I use a diesel additive and all the plugs were changed out together. It seems like it is not getting the fuel, but when you watch the plugs you can see the spray, I am looking at the electric type of solenoid coming off the battery, it seems very hot, but I am just nickel and dime myself to death. Any other suggestions?

Yes I use a diesel additive and all the plugs were changed out together. It seems like it is not getting the fuel, but when you watch the plugs you can see the spray, I am looking at the electric type of solenoid coming off the battery, it seems very hot, but I am just nickel and dime myself to death. Any other suggestions?
DO YOU KEEP IT PLUGGED UP, IF SITTING FOR LONG PERIODS OF TIME
AND HOW LONG DOES YOUR WAIT TO START LIGHT STAY ON

DO YOU KEEP IT PLUGGED UP, IF SITTING FOR LONG PERIODS OF TIME
AND HOW LONG DOES YOUR WAIT TO START LIGHT STAY ON

Yes, I keep it plugged in and the wait to start light is a couple of seconds, not to long and you can hear it clicking and doing it's thing.

Yes, I keep it plugged in and the wait to start light is a couple of seconds, not to long and you can hear it clicking and doing it's thing.
Have you had your glow plug relay checked.