Hi,
I have a 20 wk old kelpie pup that i got at 5-6 weeks. She is very one person orientated and at puppy school turned her back on the other puppies and refused to socialise with them and other people. She lives with my other kelpie and a stumpy tail cattle dog and has can't get enough of them. So it was quite a surpise that she did not like to socialise with other dogs.
Basically, i didn't push her to socialise with the other dogs as i didn't want her is start growling and nipping and i asked other people to keep their distance with their dogs until she was ready to interact with them.
A problem i find at dog training is that people don't seem to want to give your dog space and do not have enough control of their puppies or the knowledge to see that if a dog is backing off not to let you dog lunge and jump all over it.
Basically, you go to dog training to train your dog not to really socialise them after 20 weeks.
I would not be concerned. Good luck