All my puppies are poohing fine apart from one who really is struggling.
Any tips?

its a little worrying to hear its just one of the litter!
how old are the pups?
if it was diet all pups would have this problem
is it possible its eaten a part of a toy that may have caused a blockage?
hopefully this will soon pass,but you may need a vet to look at this problem

Try putting a couple of small drops of castor oil on the pups tongue. If this does not work then it would be wise to take the pup to the vets as this can be dangerous in young pups.

It went today but it does struggle. Shall take it to the vets.