there is much to learn with dogs especially pups, its not just training a dog, it is more to do with training the person who is on the other end of the leash IMO. The main goal is to be consistent. I would suggest to have the trainer work with you and have the person you leave your dog with join you. Also dogs have the ability to pick up on the slightest stress of whom ever they are paired with and will act as so.
Take when you train a dog to work protection or Shulz, what do you do? You tie the dog to a pole or a handler to hold back. This auto makes the dog more agressive to go after what is in front out of their reach. So when ever introducing someone to a dog on a leash you should always have the dog in a sit, with a command to let the dog know you are in command then let the person come in or introduce. The fist commands IMO that all should start with is Sit/down and come/here unconditional. Then move on, with out this foundation you are building on sand and not a sound foundation IMO.
I would not stress to much about this, or blame yourself. With out being there you will never know what set the dog off, and missed a chance to see the look or stance of your dog when he is stressed. It does not matter what so ome looks like or how tall, if you have your dog, they must listen to your commands as their leader. This is what you should speak about with your trainer. Start off on the small commands, with food, then play drive. Repeat, repeat, and repeat. Don't stress, stay calm in all situations, that is the hardes part. Train small and in short sports, no longer then 15 mins at a time. Stay consistency, have all who will be with the dog work the dog, to build a mutual respect. Never miss a chance to reinforce, every time doorbell rings, feeding, and all. Have fun. Hope I did not go on and on or over board. Just trying to help.