I have 6 kittens who are about 3 weeks old (no momma). They're currently on Kitten Glop as several of them stopped eating on Pro-botic, and a few more disliked KMR. They all like the kitten glop, and I'm supplementing with nutri-cal to make sure they get their vitamins.
I had to take them to the vet on Wednesday because none only 1 had pooped since I got them on Saturday. One was so constipated that she couldn't pee anymore (she's my runt). The vet helped her pee and poo, but since then it's a struggle for all of them.
I recently had several kittens w/similar problem...major constipation! Also, brought to the vet. to help w/problem. Pumpkin works well but sometimes causes gas...you could try some Slippery Elm...can find in any good health/vitamin store.
A teaspoon of pumpkin per day is what my vet recommended for an adult cat (it works on diarrhea also). Since it's healthy vegetables, I'm guessing that amount would be fine for a kitten too.
Is it possible this kitten, particularly because it's small and has no mother, still needs to be stimulated manually to go to the bathroom? I think this is done with a warm washcloth. People used to hand-raising kittens probably know the age cut-off...