From how you've written your post, it's not clear to me that you understand what Shot Start Pressure is in QuickLOAD. SSP is a variable that you enter. Hartmut put suggestions in; you can see them all if you just hover your mouse pointer over the SSP window. That will cause a help box to open and you can read his suggested pressures. You will also see that he has this parameter listed as the "pressure needed to drive bullet from case and into rifling."
All you've discovered is that when you put too little powder into the case, the total pressure QL predicts it will generate may be lower than the amount of pressure needed to drive the bullet into the rifling. Or, in other words, it's telling you you're gonna stick a bullet in the barrel.
BE WARNED: QL isn't the very best at predicting pressure, particularly when you're using powder-bullet-cartridge combos outside their 'normal' usage windows. I know you said this was just for fun; I'm re-posting this as a warning to those who may have missed the concept in the first place.
