I agree with what SableSteel said...the younger kits tend to move around more from teat to teat. But I have found exceptions with a few litters. Because I have very few rabbits and therefore only a few litters a year I am very protective of each kit that comes along and I usually end up force nursing (flipping the doe over on her back and holding her down while the babies nurse) to ensure that every kit gets their fill. Last year I had a kit who became what people here call a "fader". She was doing well up until I think about somewhere in the 1-3 weeks range and then just stopped drinking. She became uninterested in sucking. I tried everything I could think of but nothing worked. I ended up just formula feeding her until she was about 8 or 9 weeks and then weaned. She did absolutely hate it at first, but after a few days they get used to it and by the end she was actually enjoying it!
On the other hand, one of the kits in a litter I have now stopped sucking/sucked on and off like yours for about a week...think it happened when they were at about 2 weeks old. He was hungry alright but was just too lazy to suck. I ended up feeding him formula to compensate for a day or two and continued to force nurse until he did finally start sucking again after a couple days.
If your kit is a fader (even if she isn't) I would recommend formula feeding out of precaution...maybe post some pics to let us see how skinny she really is? What milk were you giving her before? I recommend this recipe:
1/2 Cup canned evaporated goat's milk, undiluted
1/2 Tablespoon corn syrup
1 Egg yolk
^^I raised a kit on that recipe and she is now almost a year old and has had 2 successful litters for me.
If she is just turning out to be lazy then I would suggest getting a teat moist with just a drop or so of water to separate the fur better and it may get her more interested. It helped a little with my little lazy boy.
Hope this helps. Fingers crossed for your kit!