
i think they all develop at different rates and what one can do others may need a few more days or so, i have had puppies really steady at 3 weeks but i have also had them unsteady still at 4 weeks, all went on to be happy healthy adults. That puppy may just be slower to develop.
If you are concerned mum may of injured her then it may be worth a check up but i hope that she is just a little slower than her littermates :)