"Normal" is for a doe to have all of the litter at one time. within a period of about 20 - 30 minutes. The does that don't generally have, um, 'issues' of some kind.
With some, it seems like they get started and just don't finish on time - one thought is that out-of-shape, flabby does may not have good enough muscle tone to get it all done at once. Genetics may play a role; a friend that bred English Angoras at one time told me that breed is notorious for spinning delivery out over a couple of days.
Sometimes, the kits themselves are the problem. An unusually large kit may take a long time to deliver, and that can happen regardless of the breeds of the parents. Generally speaking, the numerically larger the litter, the smaller the individual kits are. I have seen pedigreed Harlequin does bred to a buck of their own breed deliver single kits that were at least 6 inches long; that is larger than even Flemish Giant kits normally are. Younger does usually have kits that are all about the same size, but sometimes having kits that vary significantly in size can happen, particularly in older does.
Sometimes, kindling problems are "just one of those things," and sometimes, the animals themselves are the problem. For many years, I have held to the belief that good mothers tend to produce good mothers, so a family history of good parenting skills should be taken into consideration when choosing future breeders.
All in all, choosing not to breed this doe again may be a wise decision. As to whether they actually "mourn" lost kits, that's a bit hard to say. Rabbits certainly can form relationships with their owners and other rabbits, but my impression is that a doe's connection to her offspring seems to be more instinctual rather than emotional. They feed them, and protect them, but within a few weeks, they'll want nothing to do with them, and may even attack them.
If the doe is "sad" now, she will pretty quickly get over it.
