Some dogs/pups are just more aloof than others. I have seen it in all breeds of dogs with all sorts of socialization and cuddling when pups. One Newf pup was aloof right from 4 weeks of age, no health problems etc. He is now 5 years of age and still aloof, he comes for affection on his terms, and this started as a pup. What I do with him (and I have been bringing him around slowly since I first met him). I talk to him constantly; when I talk to him I use an extremely high pitched child like excited tone I pretend to be overly excited to see him. Example, I come into the kennel and get down to his level and say in a very excited tone, "hey puppy dog, how are you?" talk talk then when I go over to see him I go directly for his belly for a belly rub, or rub him behind his ears or under the chin...or scratch his rear end. The belly rub works for most dogs (I have met a few that don't like the belly rubbed). This is not a dominant gesture and cannot be misconstrued by the dog and put them in defense mode. I have to be extremely upbeat around him, and low and behold he is coming around and now gets excited to see me and comes at a gallop and stops short in front of me so I can rub his belly.
You can also try desentizing them by using favorite treats (I live boiled liver peices). Make of the pup, and give a few treats...the pup soon will associate good things happening when you or others are around and belly rubs will be appreciated.