There's Smoke, Now Where's the Fire?

Smoke is the most unusually-coloured of the kittens. I've never seen markings like them before. Charcoal/black on the back, tail and neck, but flip it over and you have a dark grey tabby on the legs and the mask of the face. I think Smoke might be the favourite to stay here at the moment.

Smoke worries me cos he/she isn't really thriving. Far more likely to keep sleeping than eating, it's the runt of the litter and very ribby. Whilst Sox, Stripe and Mini-mum are in training for ripping the heads off other animals, and doing it with commitment, Smoke will just be off on its own, catching up on some more shut-eye. If called upon to do so, he/she will participate in a bit off rough and tumble, but is often at the bottom of the stack.

I've tried to orchestrate a bit of Mum and Smoke alone time, but it feeds for about 30 seconds before dozing off. I keep poking Smokey's ribs to stimulate and remind him/her of what it's meant to be doing, but it then only lasts about 5 seconds.

Now I know I'm a nervous new kitten-rearer, but can anyone give me some advice or reassurance?