Bob, the problem was at least as much hard head as hard dirt. I kept ramming into a hard pile of dirt all afternoon, with the scoop on backwards. I didn't notice I'd bent the frame until I was finished. I think the problem was made worse by having slack on the two lower 3-point hookups; since then I have always filled in the extra space on the lower two hookups with large washers, leaving only 1/8" or so of slack. This prevents the jarring "ka-whump" when the hookups slide and then the tractor momentum hits at the end of the free space.
Nowadays, I have a front-end loader and no longer use the pond scoop to pick up dirt from piles, just for scraping.