She doesn’t wag her tail

My new Border collie puppy, now 12 weeks old, has been with us 3 weeks. She's the 5th we've had but it's been 13 years since our last was a puppy so I am a bit rusty. Is it normal that she doesn't wag her tail? She'll greet me when she wakes up by rolling over and giving little whines and the odd tiny lick, but no wiggling and wagging like you think of puppies as doing. She'll flick her tail slightly when visitors come so I know it CAN wag, but just not at all like I remember from the others. She's a bit on the anxious side generally, stress yawning when we carry her out for socialisation and rather too much sitting down with sad eyes and pinned back ears for my liking, but in between she's reasonably happy seeming. She'll play a bit, responds well to training, sleeps all night without a peep and is enormously enthusiastic about food! She's certainly confident when it comes to her land-shark mode! It's just a bit sad that she seems so serious and 'unsmiling'. Did anyone else have this with their pup?