Kits should not be lethargic. They should play and explore and eat well. Sometimes however thing go wrong.
Sometimes the doe will cannibalize her kits. The cause might be stress, or not enough food or water, or she might just be a cannibal. Watch for the same behavior in subsequent births and if the does continues then its time to get rid of her. Losing kits and passing on that trait is not going to get good stock.
Sometimes a doe will have her kits outside of the box. If you're lucky you'll get to them before they freeze to death. If they are born outside they may crawl out of the cage and drop to their doom. I check my does early in the morning if I can when they are expected to kindle, and as soon as I get home from work. I've only been able to successfully save one batch of kits from freezing. Death will happen, despite best intentions.
The first three days are critical for the kits, if they can get past that threshold then incidents of death from exposure are less likely.
As the kits get older, keep a mental or written log of their features, their attitudes, and anything that might be useful to monitoring their development. Make sure the doe isn't spending a lot of time in the nest box. I've had two kits crushed to death by their mothers. Its pretty grisly. Keep the box in until after the kits are weaned. If the doe can't get a respite from sucking babies she might lash out. I lost two babies that way. One's neck had been broken the other had been kicked so hard it was paralyzed and died an hour later. Its an unhappy thing to have happen especially if they had gotten through a brutal winter storm.
Everything might have been done right.Might have the right doe, buck, and the right temperatures, and calenders marked meticulously. Something might go wrong. The kits might develop diarrhea from over wet produce, or diet change or tainted water. They might develop the sniffles, a respiratory disease that causes the nose to always run and and paws to be wet and raw from wiping the moisture constantly. They might all die for no reason that can be readily apparent.
The best thing to do is keep the cages, boxes, food and water clean, mitigate where possible and let nature take care of the rest. At 2 to 3 months the babies will have put on enough weight and be ready to harvest.