> In my particular class no treats are used - once or twice I have tried it with a treat and she is there like a shot
Is that your answer then? If she's very bright she'll have sussed there is no reward for coming back in class.
I've always trained recall with treats. When first training anything I've used high value treats as a lure, til they understand what I'm asking for. Once that's been achieved I'll use them as a reward (with praise, and play too as required). When I have good and consistent behaviours I'll reduce the amount of treats and reward at random. I still always use treats for recall and never walk without something in my pocket.
In our class, when we were being tested we weren't to use treats
during the exercise, but we could reward once the exercise had finished. Do you use a clicker for training? This is quite good as you are able to 'mark' the desired behaviour then you can offer the reward after a short delay. You'll need to build up to this delay though and you must always give the treat once you have clicked.
Other things that might help are the tone of voice (high and playful) and bending down when you call her, with open arms. :)