If it’s early deaths in shell it might be egg quality. Ideally you need fresh clean eggs, all laid within a couple of days so all the same age, locally sourced and not sent through the post. Your incubator procedures were obviously good enough for half the batch to hatch, even if one didn’t make it properly, so yes, this might be a contributory cause, possibly, but not an obvious one. Early quail deaths in shell often indicate that the humidity was too high. Being so small, the highly porous shells have a greater surface area in relation to the volume of the contents than would be the case for bigger eggs with tougher thicker shells.
Having said all that, quail eggs are usually pretty tough. There are reports of people trying to incubate supermarket eggs, quite old obviously, and after a lot of disturbance, and still getting live chicks.
Where did you get yours from?