In about 1-2 seconds good Glow Plug should be getting really hot at the tip of the Glow Plug first.
If the Plugs are getting hottest at the center of the Glow plug the plug is no good.
A few Members have said after they reamed the Carbon out of the Glow Plug Holes their cold starting was easier. There is DIYs on alternative methods to clean out the holes.

Besides the Air in your Fuel System when was the last time the Injectors were checked?

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84 300D, 82 Volvo 244Gl Diesel