I agree with
@Eloy! Our birds absolutely adore nutriberries and get one per day as a bedtime snack, but when I also tried using them for clicker training they were not nearly as excited for it as I expected them to be. Same story, shorter attention spans and eventually complete disinterest in the reward. I think it has to do with the fact that while they LOVE nutriberries, on a day to day basis they are just loving them more than the pellets and chop that makes up the rest of their diet. In the grand scheme of thing however, there are still plenty of other food items that they consider more "valuable" as rewards.
So I went back to the original training treat, millet, which they ONLY get during training. And voila, instant motivation. So hopefully something like this works for you too.
Also, I you could try training a variety of tricks/commands that involve working at a station or a perch along with the recall. Things like wave, spin around, or "basketball". I find that flight recall alone can be a very "demanding" command given the reward. Generally the more energy and effort a bird has to put in, the greater the chance that they will simply decide it's not worth it after a while. Which is why trainers recommend alternating between easy, reliable tricks and more difficult ones to keep your bird motivated, especially if they are having a hard time getting it. For example, I can tell when my p'let is getting frustrated when I try to teach him to put a ring on a peg. So every 4-5 repetitions I will ask him to shake, or ring a bell. These are easy, he's got them down, and once he does it once or twice he's usually motivated to try the rings a few more times.