Most of the diesels I'm familiar with have block heaters. When warmed up with a block heater, they often start when cold even with marginal glo plugs. If you don't have a block heater, you might consider one.
Also, when is the last time you bought fuel. If the fuel is not "winterized", it will not ignite as well when cold. Most all outlets sell "winterized" fuel in the winter, but not in the summer.
Many diesels have a fuel heater that pre heats the fuel before it gets to the injector pump. It mostly helps to keep the fuel from "waxing up" when it is really cold, but it does help with cold starts. If your engine has one, is it working?