Honestly, it's really not necessary to weigh them all the time. I've never weighed mine in the past 3 1/2 years of raising rabbits. Don't see the need in it, unless I was raising them for show. Their weights will vary from day to day. Every now and again one may miss a feeding. One of my moma's had 8 kits and I noticed, over several days, that 2 were getting thin. I also found some outside of the box where they had hung on to moma's tit as she jumped out of box and ended up on the wire, overnight, thus missing its morning feeding. Could be a number of reasons. Moma's usually feed early in the morn and in the evening and they're not in the box that long. Just keep your eye's on them like you've been doing. If you see one getting exceptionally thin, that's when you intervene, as you mentioned. In my case, I, fortunately, had another moma that had kits at the same time as the other, but she only had 6, so I put the 2 thin ones in with her until they fattened up a bit, maybe 4 days, and then put them back with they're moma. They all turned out well.