She just desn't seem to be catching up, she is only putting on tiny amounts now and is just over half the weight of the biggest ones (600g compared to 1050g)
We are taking her out separately as suggested but she seems to mess with the teats a lot, gets on has a few sucks then slips off and fiddles around trying to get on again.
We started all of them on baby rice and puppy replacement milk yesterday and I am giving Twiglet a bit in the morning also. Is it worth continuing the nutri drops and how often as she usually spits most of it out?
They are 15 days now and  am very proud of Pippa for managing to feed all 12 of them, the rest all seem fat and contented.
Please can I stop worrying or am I right to be concerned?
My mentor says she thinks she is fine and will be ok.
Thanks.

I agree with your mentor, she is now 15 days and the only problem she is having is being smaller than her siblings. I wouldn't bother with the nurtidrops, others may disagree.

Is she lapping up the replacement milk?  And she is eating a little puppy rice?  If so she is just taking in whatever she can, being small it will not be anywhere near as much as the others. If you feel she is trying to suckle but not getting anywhere and not lapping up the replacement milk, then a shringe feed of the replacement milk may be worth adding, but be careful not to overfeed her, I would feed her half the recommended dose if she is half the normal weight for her age.

Have a search for Gwen and her little pup not so long ago, she was very tiny, but also behind on her feeding which your pup does not seem to have a problem with, but the thread may well make you feel more at ease with this pup. :-)

I would suggest that you just continue as you are.  Just make sure that the little one has extra top ups from mum, little and often. You can take pup out of the whelping box and on the floor with mum when mum is laying near you.  Once you get properly into weaning you will be suprised how quickly she comes on. 

If she's not latching on for long and fussing at the nipple, try putting one of the lusty pups on a good nipple for a few seconds to get the milk flowing and then put her on that nipple instead, it just might help her to get the extra she needs ;)   Don't forget, she'll put weight on in line with her present weight, try checking how much the others were putting on at her weight - you maybe surprised ;)

Yes, try like Dill says putting a big pup on then switching them over. But basically as long as she is gaining and is ok in herself, she'll probably be fine. You might find she leaps ahead once they're weaned! :-)

is this her first litter. I've got a bitch who has had 3 litters, and in each litter she had a tiny bitch pup. They all grew up to be normal healthy dogs, just a bit on the small side. I kept the small one from her last litter as, though she is a bit too small to show, she is almost perfect in every other way. I have bred from her and all her puppies were normal size.

I had a very small (5oz) bitch in my first litter (less than half the size of the others).
She was very healthy. When I saw her again at 13 months old she was huge, much bigger than the bitch I kept (who happened to be of the largest). She had not been spayed so that wasn't the reason she became so big. As long as she is feeding and putting weight on I wouldn't be overly worried.

Thank you for all your support/advice/experience.
She seems to be gaining steadily again and reached over 700g now. It will be interesting to see how she grows up compared to the others as lots of people are saying she won't necessarily be the littlest for ever.
She certainly has a personality and is busy paper training herself LOL!