I need help kitten wont eat!!!?

Hi i really need some help, my cat had a litter of kittens a week ago today, all are doing great except one - it wont eat atall, ive tried latching it onto its mother but nothing, the mother isnt rejecting it as she is still cleaning it and cuddling it in etc, you can really tell the difference to all the other kittens as she is tiny and all others have started opening their eyes where she hasnt. Today i bought a tin of Lactol and a feeding kit to try and build her up and encourage her to suckle on mother but she isnt interested in that either - she's just yelping, all she wants to do is sleep, anyone got any other suggestions as i really dont want to loose this kitten, thankyou