I use the lowest heat possible and Easy silver solder but I have
problems keeping the Bi-metal from "cupping" as it heats.
This is just a guess, as I've not actually used this stuff. But the
problem, as I'm sure you know, is that the two layers expand at
different rates from heating. It occurs to me that if you're using
too easy a grade of solder, you may not be getting the metal hot
enough to reach annealing temps. Try heating it a little hotter, to
be sure it's fully annealed at the point where the solder is to flow.
I don't know if this will work, but perhaps it will help, since this
way, at least, you know the metal is soft, and able to redistribute
and eliminate stresses in the metal which are forcing out of flat.
What I don't know is whether it will at this point relax again and
flatten out for you. It will, after all, at temperature still have
expanded differently on one side than the other. But it might,
nevertheless, work. Worth a try, I think.