Normally, the 6.2/6.5 doesn't need glow to start if the engine temperature is above about 80 degrees F.

Those that do have some sort of problem. Injection timing, worn fuel injection pump/injectors that produce improperly atomized fuel, low cranking speed, thin fuel (i.e. #1 diesel in warmer temps), low cylinder compression, low cetane fuel, and so on... These are the most likely.