My GCC was having a bit of trouble with fetch as well. She's on the small side, even for a GCC, so I couldn't find a good prop for her to use. Michael used a quarter for his cape in his video, so I tried a dime, but she couldn't hang onto that for long because it was so flat, and as we were just starting out, she would take it from me and then drop it right away, probably on accident...into the cup I was holding right under her. So she started thinking that she was supposed to just grab the thing for a split second and drop it right away and was doing that even after I found a better prop (colorful pellets from the seed mix I use for treats--haha). I think it's definitely important to make sure the prop is easy for the bird to grasp, and that it's not dropping it just because it cant hang on anymore, but because it wants to. I just kept going at it, sometimes holding the cup away from her on purpose to see if she'd walk over.
ANYWAY, I don't know if this is an ACTUAL useful strategy, because I did it kind of by accident, and I'm not positive this is what helped my bird to learn the trick, but...eventually I got tired of turning to grab a treat from behind me every time and started keeping them IN the target cup. Soon she realized that's where they were, and would move toward the cup in the hopes of retrieving a treat. I had to not bring the cup into range until she was already holding the prop (otherwise she'd go for the cup and ignore the prop), but soon she would be walking towards the cup, prop in beak, and then as she reached in to try to grab a seed (I soon stopped keeping them in there), drop the prop into the cup in the process. If you try this, you might want to make sure your bird is still always getting the treat from you and not picking it out of the cup herself.
Long winded, but that's what worked for me. I'm not sure it wasn't just a fluke, but its something you could try as well. If your bird starts getting too eager to go to the cup and ignores the prop, try keeping the cup out of reach until its got a hold on the prop. Another thing I did, to questionable effect, was miming fetching myself while my bird watched. Good luck!