Ok, from your description I'm of course just making an educated guess from hatching entirely too many chicks over the years, lol.
Things to consider here, humidity is a tool just to allow for the eggs to lose the appropriate amount of weight. Too high and too low both have unfortunate complications.
As hens age, their eggs become more porous even when given plenty of calcium, so their eggs will lose weight more quickly.
I have experimented with many different incubators and they all have a sweet spot. For me in the NR360 it's around 45% for the first 18 days and increasing to 65% for hatch. These are the percentages that I have very rarely needed to adjust when closely monitoring weight loss no matter the breeds or age of the hens so I feel very comfortable suggesting those percentages even if you aren't on top of monitoring air cell growth or weight.
The fact that you have healthy development and then lose them, I think this is the most likely culprit. That's not to say I haven't encountered a bizarre genetic issue that caused a high occurrence of late quitters but it was one time, with one flock, out of soooo many hatches.
I hope this helps!