Oh, you hit the nail on the head. It's definitely a case of individual sensitivities. My first ekkie, patches was a complete chow hound. Not so much with a lot of veggies, but any fruit in front of him and he was in his glory. I didn't worry about the extra calories since he was pretty naked, so he needed it to keep warm and maintain body weight. Nothing bothered him, and he never ever toe tapped.
Fast forward to Tucker... He's definitely sensitive to certain things. Process of elimination, corn, even in small amounts, and dark purple grapes caused him to tap badly, and took more than a week to completely subside once I figured it out.
Now I have my cute little grey, who's also somewhat plucked on his chest, burns tons of calories, I'm back to offering higher calorie stuff like fruits, and seeing what he likes as he's still quite new to me. He was losing weight soon after I got him, despite the improved diet that he took to very easily.
Anyway, a couple weeks ago, I thought I'd let Tucker have a treat since I was offering it to the other two. A couple chunks of mango which he loved, and a blueberry that he didn't even finish for breakfast. Bad idea. By supper time, he started tapping, and next morning it was to the point of about every 2-3 seconds, and periodically kicking one leg out behind him and shaking/scratching/scraping on the floor of the cage. I realized what I had done, and sat there and cried for my poor boy, and my own stupidity. I screwed up, and my sweet Tucker paid for it. Despite always trying our best, once in a while I manage to feel like a total failure, even though there's no way I could have known. It's humbling for sure. It again took over a week to completely clear.