That might be your problem, the fact that she is not getting a reward for her to come to you. The problem is you don't know anything about her past or how she was trained or treated so you have to come into this expecting the worst , no one really gives up a perfectly behaved dog. It is likely this dog broke free a lot and was punished for it, though she may have very well also been trained on leash so she knows she cannot get away and has no choice to go to you. However her reward is only praise so what is the payoff?
You have trained two therapy dogs (I was an evaluator for tdi for many years) but I am suspecting their background history was very different and what works for one breed does not always work for another and the same goes from dog to dog.
It is very important to see what will motivate this dog to turn around and come back to you. I might start putting something special in his food and call him to come to you at each feeding. I might also change the word come to here in case there are bad connotations associated with the word come from his past.
I would start off at home in a small area off leash and practice come in close proximity with a special treat in your hand such as a hot dog or chicken sliver, then increase your distance and wait time.
It is clear that this cue is not being made special enough that he wants to come to you.
I might also use a different command altogether such as heel or side and see if he will come back to you for that off leash. If he does not then you need to up your game and find something that floats his boat.