I used my utility knife to re-scrape the back 1/4" or so of each scrape on two different reeds (one by me and one by Pudinka), in two stages. First I widened the point of each scrape from a "V" to more of a "U". This in itself didn't do the trick: apparently I didn't remove enough enough material.
So, on each scrape I then widened the "U" a little further out each way, scraped down a little harder to remove more material, and finished with a few scrapes forward towards the thinner part of the scrape, in order to smooth the transition. I did all this with bridles off (except testing the reeds between two stages), and without any sandpaper.
Each reed now has a C-natural that locks in right away, in tune and stable. Success! Hearty thanks to Andy, and to the rest of ye's!
I'm taking away two lessons from all of this. First, when sanding, I should try to keep most of the pressure back towards the apex of the scrape. Even my best reeds seem to end up needing to be trimmed and re-sanded before they'll play properly (regardless of how C-natural turns out), so I think I tend to over-sand the tips.
Second, I'm beginning to think it's best to scrape more and sand less. Are many of you using Koehler's "scrapers only" method? I read the "Koehler gouge" thread with interest, but I'm reluctant to completely re-tool at this point.
Footnote: the Pudinka reed I re-scraped, made of his "hard, mean Texan cane," now plays very well (and loudly) with no bridle on. Props to Tim Britton?
We'll see how long that lasts, though -- Minnesota weather shifts radically and often, and summer's hardest of all on pipes because A/C is so much drier than the ultra-humid air outside. [sigh]
