is this always the same? If so, she probably by some reason thinks he wont make it, and thus abandon it to give all her might to those whom its hope for.Also, something strange: This is the third time I've found one or more of the kittens outside of the box. Why would she move them? I don't want them on the cold, hard floor.
If so, you must probably handraise it, if you want to give it a chance.
If they are different each time, this is another reason: I bet they are pushing and fighting at the nipples... She finds it bothersome, and thus, make her own private rotation. Some good moms take even away one, and feed it outside, next time someone else.
So your best course is to help her with the rotation.. 2on 3 off, and switch. Make an extra little nest with heating pad and furry toy companion, where these on off watch may take their midday nap.
The smallest may theoretically add somewhat less, perhaps 8+, and its still OK. But I have seen many minors whom added as if they were twice as big in bithing weigh...