Sorry to hear that, such a shame. Amazon Milk Frogs are actually a very hardy frog, but their needs have to be met like an any other animal. Sounds certain to me that this didn't happen with these guys and there would have been very little you could have done. Most likely something like MBD caused by lack of calcium and UVB.
If this experience hasn't put you off, come and see me at the November show. I'll have some Milks on my table that have been raised correctly.