So we currently have a 6 month old female GSD mix. We got her from a rescue when she was 12 weeks. She was fine upon visiting her at the rescue- we have had dogs our whole life and know a thing or 2 about them
There were no red flags. We got her home and we didn't see her for a few days. She was either under the table or behind her crate. We have 2 other dogs which helped relax her in our house. For the first few days, we stayed home with her so she could get acclimated. Well 4 days later our 16 year old son comes home from camp. And she low growled at him. Seriously. I was so upset. We had a local trainer come and analyze what she thought of the situation. She said our puppy is not aggressive, she is reacting out of fear. Startled fear.
You see, we have now had her 13 weeks and she still occasionally growls at our son but the 3 of us have worked so hard to desensitize her from the fear stage. You see, my son is a "loud" 16 year old- loud in the hose, loud in the bathroom, loud walking around, loud closing the fridge. She had to be desensitized from many sounds.
These are some of the tings we have done:
1. If someone walking into a room emits a growl from her (usually my son), I would role play that activity several times that day and the next few mornings. She would think it would be my son, but then she'd be shocked to see it was me or hubby and eventually she stopped growling at someone entering the room because it wasn't always my son. I would drag my feet, slam doors, slam fridge, and she would come running up to me and then quietly walk away.
2. When my son is home we have conversations in his room, on his bed, and I would bring in the other dogs, leaving it up to Piper whether or not she wanted to join us. Which she does now, all the time. She will even jump on his bed.
3. We have our son feed her when he's home- he has a job, school, and friends- so hard to catch him sometimes.
4. We almost take her somewhere in the car every day. Sometimes, my son joins us, sometimes not. We take her to the beach, Home Depot, Lowe's, Petco, etc. Only for 10-15 minutes but it's something.
5. When my son is on his way home from school or work he lets me know and I get out the spray cheese. Yes, spray cheese. Nasty tasty stuff to dogs. Everytime our son comes in, all dogs get cheese. It's a way that Piper looks forward to him coming home- she used to growl when he would come in the house. Now, we try not to give her any reason to growl.
6. On her leash we always treats in a fanny pack with us to give her so we keep her attention regardless if a dog or people walk by.
7. She doesn't like it when people come up to her but she will go up to you on her terms on her time.
8. Piper is crated when we aren't home and my husband and I are the only ones allowed to put her in "jail"; we have let our son free her but sometimes she growls (who knows why???) so now we handle the crates so again, no reason for her to growl.
My husband and I have an awesome relationship with her- she is a "normal" dog with us. And she is getting better and better with our son. It has been a journey and we have been exhausted and frustrated but we have kept at it. Piper has come a LONG way and we are so proud of what we have done with her. It is a work in progress and I don't know if we will ever stop the nasty tasty spray cheese but if it keeps the peace....
Our son can play ball with her out back, he can pet her, she goes in his room and says good morning but she still keeps her distance. It's not all her fault. When there has been opportunity, he has chosen to be on his phone or ignore her because football is on TV. But he's a teenager and he didn't ask for her to growl at him.
I know our situation isn't the exact same as yours, but maybe something I wrote will help.