It would be helpful to try call apt install <not-upgradable-package> manually. In this case apt should show a problem that blocked the package to upgrade.
Of cource, all packages must be already downloaded. You could use --download-only option for that.
For one of my testing setup (the first Crimson image upgradeg to backports) FF lost their interface. --safe-mode and new profile solved the problem for some time, but finally it fail back to the same state. I have no idea whet it is… Fortunately, upgraded Byzantium works well.