How exactly did you first start teaching the come command? I like positive reinforcement and would recommend you stick to it, but that doesn't mean the trainer you have is actually any good, she might be, she might not be. Not all positive trainers are equal.
When you first teach this command you should find a way to get the dog to come to you without saying come or the dog's name and without using a food or toy lure. You can make funny noises, you can run away (that usually works well for most dogs), you can sit down on the floor, lie down on the floor, pant (dogs sometimes get concerned when people do this), go stand in the corner, jump up and down, imitate a play bow, spin in circles, anything that will make the dog come over and investigate (and that you won't be too embarrassed to do out in the yard lol). When they do come over you have a party, lots of treats. Then run off and do it again. This should begin inside. You should not use the come command at all until the dog is proficient at it in whatever environment you want to use it in.
So you practice having the dog coming to you inside with your body language or funny noises. When the dog is coming over and over whenever you do the chosen action you can add the come command. So say "come!" and then do whatever action was making the dog come. Then give the treats when she gets to you. The order is important, say the command before doing the action. What you want to happen is for the dog to figure out that the word come always happens before you run away pr do whatever action, and just anticipate that and come before you do it. So give a brief pause between saying come and doing the action so you can see if she starts moving before the action at all. Once she starts anticipating and coming after the command but before you do any action you can usually stop doing the action. At the next training session she may regress a bit and need the action the first few times, but it'll be quick. Remember to keep all training sessions pretty short, you don't want to bore her. If you play tug you can incorporate the training into those games. When you win the tug run off and as she's chasing you to come get it call "come" and turn to let her run to you and continue the game. Make sure to end the game before she decides she's done. You never want to call her when she's not going to come in these early stages.
Through all of this the treats should be in your pocket. Never in your hand, they come out either when she gets to you or when she's already well on her way. I suggest that you stash treats in containers around your house or carry treats in your pockets at all times so you can always randomly plunge into a training session, she doesn't watch you go get treats first. She never knows when you have treats, you could have them at any time, so she should listen to that command, it could pay off. In the early stages never call her to come unless you can reward her. You can call her to come, clip a leash and immediately go for a walk, that's a reward (if she likes walks). It doesn't have to always be treats (they just work well in sessions because you can use them over and over) but it needs to be something the dog really likes. If you're cooking and drop food one the floor that's safe for her call her to come and direct her to it (if you think she'll like it...so not lettuce or something like that). Come should be very profitable.
Sometimes when you call her to come, even when she knows the command, be fun. Run away, be silly, don't always just stand there like a pole, poles are boring.
So work on that for now,reteach the command, make come more fun, stop having treats in your hand, and start having treats all over the place so she thinks she could always get one, whether she saw it or not. We have treats in our fridge for when my dog listens in the kitchen and a container in the living room for when he listens in there. If we go outside I put some in my pocket. When I ask my dog to do something in the living room I praise him for doing it and make my way to the treat container. The dog learns that praise means a reward is going to happen, so he gets excited when you start praising and you get the reward ASAP. Don't make any move towards the container until after the dog is already coming or has already gotten to you. Otherwise you'll teach the dog to check whether or not your going to get a treat before coming to you.
Much later in training you'll be able to do varied reinforcement (reward with treats sometimes, only praise/petting other times) but not for a long while.
I also suggest reading The Other End of the Leash by Patricia McConnell. She talks about training her Great Pyr, it might be helpful to you.
