Does first litter and only one kit survived, why?

by Nicole
(Milton Keynes , England , U.K. )

Hi,

My doe has recently had litter and only one kit has survived. I originally thought she was
Pregnant but after a few days of pulling fur and what looked like nesting to me she stopped,
I moved her into a smaller separate cage from her mate and set up a nesting box put all her fur she
Had pulled along with some hay and straw ect ... Thinking that she was still pregnant, a week passed
And nothing happened so I began to think that maybe she wasn't pregnant after all she didn't look fat atall and I attempted to palpate but didn't want to hurt incase she was so didn't get very far with that. Then my doe and her mate were pining for each other so I put her back in with him as at this point I didn't think she was pregnant.
Two days ago we found a kit big tummy looks very healthy and had fur I think it must have been 3/4 days old I felt terrible I hadn't noticed sooner as they are house rabbits they have their own room I get them out every day feed them ect... but I hadn't checked their bed as we don't usually clean that out just add more bedding! I didn't want to disturb the nest she had made although tried to check for other baby's I originally found two dead baby's with next to no fur and very tiny and a day later another the same hidden amongst some fur and bedding I feel so horrible that I hadn't noticed sooner could I have done something to save them? Why hadn't they survived ? The one that was alive was in the nest, two were in the downstairs of the hutch that had died and the other one just outside the nest area which had also died? They didn't look injured atall both mum and dad bunnies are very placid but dad had been in there until I found them at day 3/4 I took him out immediately though and the room the bunnies are in is very warm warmest room in the house.
I am distraught thinking what if I could've done something and basically just want to know what could've happened ? Why possibly did they die?

Thanks for reading

From a very upset Nicole